[PATCH RESEND v3 0/4] lib: introduce copy_struct_from_user() helper

[PATCH RESEND v3 0/4] lib: introduce copy_struct_from_user() helper

[Aleksa Sarai]

From: Aleksa Sarai <cyphar@cyphar.com>

Patch changelog:
 v3: [<https://lore.kernel.org/lkml/20190930182810.6090-1-cyphar@cyphar.com/>]
  * Rename is_zeroed_user() to check_zeroed_user(). [Christian Brauner]
  * Various minor cleanups. [Christian Brauner]
  * Add copy_struct_from_user() tests.
 v2: <https://lore.kernel.org/lkml/20190925230332.18690-1-cyphar@cyphar.com/>
 v1: <https://lore.kernel.org/lkml/20190925165915.8135-1-cyphar@cyphar.com/>

This series was split off from the openat2(2) syscall discussion[1].
However, the copy_struct_to_user() helper has been dropped, because
after some discussion it appears that there is no really obvious
semantics for how copy_struct_to_user() should work on mixed-vintages
(for instance, whether [2] is the correct semantics for all syscalls).

A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases).

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[3]). This series
implements the helper and ports several syscalls to use it.

Some in-kernel selftests are included in this patch. More complete
self-tests for copy_struct_from_user() are included in the openat2()
patchset.

[1]: https://lore.kernel.org/lkml/20190904201933.10736-1-cyphar@cyphar.com/

[2]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

[3]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Aleksa Sarai (4):
  lib: introduce copy_struct_from_user() helper
  clone3: switch to copy_struct_from_user()
  sched_setattr: switch to copy_struct_from_user()
  perf_event_open: switch to copy_struct_from_user()

 include/linux/bitops.h     |   7 ++
 include/linux/uaccess.h    |   4 ++
 include/uapi/linux/sched.h |   2 +
 kernel/events/core.c       |  47 +++----------
 kernel/fork.c              |  34 ++--------
 kernel/sched/core.c        |  43 ++----------
 lib/strnlen_user.c         |   8 +--
 lib/test_user_copy.c       | 133 +++++++++++++++++++++++++++++++++++--
 lib/usercopy.c             | 123 ++++++++++++++++++++++++++++++++++
 9 files changed, 287 insertions(+), 114 deletions(-)

-- 
2.23.0

[PATCH RESEND v3 1/4] lib: introduce copy_struct_from_user() helper

[Aleksa Sarai]

From: Aleksa Sarai <cyphar@cyphar.com>

A common pattern for syscall extensions is increasing the size of a
struct passed from userspace, such that the zero-value of the new fields
result in the old kernel behaviour (allowing for a mix of userspace and
kernel vintages to operate on one another in most cases).

While this interface exists for communication in both directions, only
one interface is straightforward to have reasonable semantics for
(userspace passing a struct to the kernel). For kernel returns to
userspace, what the correct semantics are (whether there should be an
error if userspace is unaware of a new extension) is very
syscall-dependent and thus probably cannot be unified between syscalls
(a good example of this problem is [1]).

Previously there was no common lib/ function that implemented
the necessary extension-checking semantics (and different syscalls
implemented them slightly differently or incompletely[2]). Future
patches replace common uses of this pattern to make use of
copy_struct_from_user().

Some in-kernel selftests that insure that the handling of alignment and
various byte patterns are all handled identically to memchr_inv() usage.

[1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

[2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
     similar checks to copy_struct_from_user() while rt_sigprocmask(2)
     always rejects differently-sized struct arguments.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/linux/bitops.h  |   7 +++
 include/linux/uaccess.h |   4 ++
 lib/strnlen_user.c      |   8 +--
 lib/test_user_copy.c    | 133 ++++++++++++++++++++++++++++++++++++++--
 lib/usercopy.c          | 123 +++++++++++++++++++++++++++++++++++++
 5 files changed, 262 insertions(+), 13 deletions(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index cf074bce3eb3..c94a9ff9f082 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -4,6 +4,13 @@
 #include <asm/types.h>
 #include <linux/bits.h>
 
+/* Set bits in the first 'n' bytes when loaded from memory */
+#ifdef __LITTLE_ENDIAN
+#  define aligned_byte_mask(n) ((1UL << 8*(n))-1)
+#else
+#  define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
+#endif
+
 #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
 #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
 
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 70bbdc38dc37..94f20e6ec6ab 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -231,6 +231,10 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
 
 #endif		/* ARCH_HAS_NOCACHE_UACCESS */
 
+extern int check_zeroed_user(const void __user *from, size_t size);
+extern int copy_struct_from_user(void *dst, size_t ksize,
+				 const void __user *src, size_t usize);
+
 /*
  * probe_kernel_read(): safely attempt to read from a location
  * @dst: pointer to the buffer that shall take the data
diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
index 28ff554a1be8..6c0005d5dd5c 100644
--- a/lib/strnlen_user.c
+++ b/lib/strnlen_user.c
@@ -3,16 +3,10 @@
 #include <linux/export.h>
 #include <linux/uaccess.h>
 #include <linux/mm.h>
+#include <linux/bitops.h>
 
 #include <asm/word-at-a-time.h>
 
-/* Set bits in the first 'n' bytes when loaded from memory */
-#ifdef __LITTLE_ENDIAN
-#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
-#else
-#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
-#endif
-
 /*
  * Do a strnlen, return length of string *with* final '\0'.
  * 'count' is the user-supplied count, while 'max' is the
diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
index 67bcd5dfd847..3a17f71029bb 100644
--- a/lib/test_user_copy.c
+++ b/lib/test_user_copy.c
@@ -16,6 +16,7 @@
 #include <linux/slab.h>
 #include <linux/uaccess.h>
 #include <linux/vmalloc.h>
+#include <linux/random.h>
 
 /*
  * Several 32-bit architectures support 64-bit {get,put}_user() calls.
@@ -31,14 +32,129 @@
 # define TEST_U64
 #endif
 
-#define test(condition, msg)		\
-({					\
-	int cond = (condition);		\
-	if (cond)			\
-		pr_warn("%s\n", msg);	\
-	cond;				\
+#define test(condition, msg, ...)					\
+({									\
+	int cond = (condition);						\
+	if (cond)							\
+		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
+	cond;								\
 })
 
+static bool is_zeroed(void *from, size_t size)
+{
+	return memchr_inv(from, 0x0, size) == NULL;
+}
+
+static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
+{
+	int ret = 0;
+	size_t start, end, i;
+	size_t zero_start = size / 4;
+	size_t zero_end = size - zero_start;
+
+	/*
+	 * We conduct a series of check_nonzero_user() tests on a block of memory
+	 * with the following byte-pattern (trying every possible [start,end]
+	 * pair):
+	 *
+	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
+	 *
+	 * And we verify that check_nonzero_user() acts identically to memchr_inv().
+	 */
+
+	memset(kmem, 0x0, size);
+	for (i = 1; i < zero_start; i += 2)
+		kmem[i] = 0xff;
+	for (i = zero_end; i < size; i += 2)
+		kmem[i] = 0xff;
+
+	ret |= test(copy_to_user(umem, kmem, size),
+		    "legitimate copy_to_user failed");
+
+	for (start = 0; start <= size; start++) {
+		for (end = start; end <= size; end++) {
+			size_t len = end - start;
+			int retval = check_zeroed_user(umem + start, len);
+			int expected = is_zeroed(kmem + start, len);
+
+			ret |= test(retval != expected,
+				    "check_nonzero_user(=%d) != memchr_inv(=%d) mismatch (start=%zu, end=%zu)",
+				    retval, expected, start, end);
+		}
+	}
+
+	return ret;
+}
+
+static int test_copy_struct_from_user(char *kmem, char __user *umem,
+				      size_t size)
+{
+	int ret = 0;
+	char *rand = NULL, *expected = NULL;
+	size_t ksize, usize;
+
+	rand = kmalloc(size, GFP_KERNEL);
+	if (ret |= test(rand == NULL, "kmalloc failed"))
+		goto out_free;
+
+	expected = kmalloc(size, GFP_KERNEL);
+	if (ret |= test(expected == NULL, "kmalloc failed"))
+		goto out_free;
+
+	/* Fill umem with random bytes. */
+	memset(kmem, 0x0, size);
+	prandom_bytes(rand, size);
+	ret |= test(copy_to_user(umem, rand, size),
+		    "legitimate copy_to_user failed");
+
+	/* Check basic case -- (usize == ksize). */
+	ksize = size;
+	usize = size;
+	memcpy(expected, rand, ksize);
+
+	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
+		    "copy_struct_from_user(usize == ksize) failed");
+	ret |= test(memcmp(kmem, expected, ksize),
+		    "copy_struct_from_user(usize == ksize) gives unexpected copy");
+
+	/* Old userspace case -- (usize < ksize). */
+	ksize = size;
+	usize = ksize / 2;
+
+	memcpy(expected, rand, usize);
+	memset(expected + usize, 0x0, ksize - usize);
+
+	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
+		    "copy_struct_from_user(usize < ksize) failed");
+	ret |= test(memcmp(kmem, expected, ksize),
+		    "copy_struct_from_user(usize < ksize) gives unexpected copy");
+
+	/* New userspace (-E2BIG) case -- (usize > ksize). */
+	usize = size;
+	ksize = usize / 2;
+
+	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize) != -E2BIG,
+		    "copy_struct_from_user(usize > ksize) didn't give E2BIG");
+
+	/* New userspace (success) case -- (usize > ksize). */
+	usize = size;
+	ksize = usize / 2;
+
+	memcpy(expected, rand, ksize);
+
+	ret |= test(clear_user(umem + ksize, usize - ksize),
+		    "legitimate clear_user failed");
+	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
+		    "copy_struct_from_user(usize > ksize) failed");
+	ret |= test(memcmp(kmem, expected, ksize),
+		    "copy_struct_from_user(usize > ksize) gives unexpected copy");
+
+out_free:
+	kfree(expected);
+	kfree(rand);
+	return ret;
+}
+
 static int __init test_user_copy_init(void)
 {
 	int ret = 0;
@@ -106,6 +222,11 @@ static int __init test_user_copy_init(void)
 #endif
 #undef test_legit
 
+	/* Test usage of check_nonzero_user(). */
+	ret |= test_check_nonzero_user(kmem, usermem, 2 * PAGE_SIZE);
+	/* Test usage of copy_struct_from_user(). */
+	ret |= test_copy_struct_from_user(kmem, usermem, 2 * PAGE_SIZE);
+
 	/*
 	 * Invalid usage: none of these copies should succeed.
 	 */
diff --git a/lib/usercopy.c b/lib/usercopy.c
index c2bfbcaeb3dc..cf7f854ed9c8 100644
--- a/lib/usercopy.c
+++ b/lib/usercopy.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/uaccess.h>
+#include <linux/bitops.h>
 
 /* out-of-line parts */
 
@@ -31,3 +32,125 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
 }
 EXPORT_SYMBOL(_copy_to_user);
 #endif
+
+/**
+ * check_zeroed_user: check if a userspace buffer only contains zero bytes
+ * @from: Source address, in userspace.
+ * @size: Size of buffer.
+ *
+ * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
+ * userspace addresses (and is more efficient because we don't care where the
+ * first non-zero byte is).
+ *
+ * Returns:
+ *  * 0: There were non-zero bytes present in the buffer.
+ *  * 1: The buffer was full of zero bytes.
+ *  * -EFAULT: access to userspace failed.
+ */
+int check_zeroed_user(const void __user *from, size_t size)
+{
+	unsigned long val;
+	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
+
+	if (unlikely(size == 0))
+		return 1;
+
+	from -= align;
+	size += align;
+
+	if (!user_access_begin(from, size))
+		return -EFAULT;
+
+	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+	if (align)
+		val &= ~aligned_byte_mask(align);
+
+	while (size > sizeof(unsigned long)) {
+		if (unlikely(val))
+			goto done;
+
+		from += sizeof(unsigned long);
+		size -= sizeof(unsigned long);
+
+		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
+	}
+
+	if (size < sizeof(unsigned long))
+		val &= aligned_byte_mask(size);
+
+done:
+	user_access_end();
+	return (val == 0);
+err_fault:
+	user_access_end();
+	return -EFAULT;
+}
+EXPORT_SYMBOL(check_zeroed_user);
+
+/**
+ * copy_struct_from_user: copy a struct from userspace
+ * @dst:   Destination address, in kernel space. This buffer must be @ksize
+ *         bytes long.
+ * @ksize: Size of @dst struct.
+ * @src:   Source address, in userspace.
+ * @usize: (Alleged) size of @src struct.
+ *
+ * Copies a struct from userspace to kernel space, in a way that guarantees
+ * backwards-compatibility for struct syscall arguments (as long as future
+ * struct extensions are made such that all new fields are *appended* to the
+ * old struct, and zeroed-out new fields have the same meaning as the old
+ * struct).
+ *
+ * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
+ * The recommended usage is something like the following:
+ *
+ *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
+ *   {
+ *      int err;
+ *      struct foo karg = {};
+ *
+ *      if (usize > PAGE_SIZE)
+ *        return -E2BIG;
+ *      if (usize < FOO_SIZE_VER0)
+ *        return -EINVAL;
+ *
+ *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, usize);
+ *      if (err)
+ *        return err;
+ *
+ *      // ...
+ *   }
+ *
+ * There are three cases to consider:
+ *  * If @usize == @ksize, then it's copied verbatim.
+ *  * If @usize < @ksize, then the userspace has passed an old struct to a
+ *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
+ *    are to be zero-filled.
+ *  * If @usize > @ksize, then the userspace has passed a new struct to an
+ *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
+ *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
+ *
+ * Returns (in all cases, some data may have been copied):
+ *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
+ *  * -EFAULT: access to userspace failed.
+ */
+int copy_struct_from_user(void *dst, size_t ksize,
+			  const void __user *src, size_t usize)
+{
+	size_t size = min(ksize, usize);
+	size_t rest = max(ksize, usize) - size;
+
+	/* Deal with trailing bytes. */
+	if (usize < ksize) {
+		memset(dst + size, 0, rest);
+	} else if (usize > ksize) {
+		int ret = check_zeroed_user(src + size, rest);
+		if (ret <= 0)
+			return ret ?: -E2BIG;
+	}
+	/* Copy the interoperable parts of the struct. */
+	if (copy_from_user(dst, src, size))
+		return -EFAULT;
+	return 0;
+}
+EXPORT_SYMBOL(copy_struct_from_user);
-- 
2.23.0

[PATCH RESEND v3 2/4] clone3: switch to copy_struct_from_user()

[Aleksa Sarai]

From: Aleksa Sarai <cyphar@cyphar.com>

The change is very straightforward, and helps unify the syscall
interface for struct-from-userspace syscalls. Additionally, explicitly
define CLONE_ARGS_SIZE_VER0 to match the other users of the
struct-extension pattern.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 include/uapi/linux/sched.h |  2 ++
 kernel/fork.c              | 34 +++++++---------------------------
 2 files changed, 9 insertions(+), 27 deletions(-)

diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index b3105ac1381a..0945805982b4 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -47,6 +47,8 @@ struct clone_args {
 	__aligned_u64 tls;
 };
 
+#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
+
 /*
  * Scheduling policies
  */
diff --git a/kernel/fork.c b/kernel/fork.c
index f9572f416126..2ef529869c64 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2525,39 +2525,19 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
 #ifdef __ARCH_WANT_SYS_CLONE3
 noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
 					      struct clone_args __user *uargs,
-					      size_t size)
+					      size_t usize)
 {
+	int err;
 	struct clone_args args;
 
-	if (unlikely(size > PAGE_SIZE))
+	if (unlikely(usize > PAGE_SIZE))
 		return -E2BIG;
-
-	if (unlikely(size < sizeof(struct clone_args)))
+	if (unlikely(usize < CLONE_ARGS_SIZE_VER0))
 		return -EINVAL;
 
-	if (unlikely(!access_ok(uargs, size)))
-		return -EFAULT;
-
-	if (size > sizeof(struct clone_args)) {
-		unsigned char __user *addr;
-		unsigned char __user *end;
-		unsigned char val;
-
-		addr = (void __user *)uargs + sizeof(struct clone_args);
-		end = (void __user *)uargs + size;
-
-		for (; addr < end; addr++) {
-			if (get_user(val, addr))
-				return -EFAULT;
-			if (val)
-				return -E2BIG;
-		}
-
-		size = sizeof(struct clone_args);
-	}
-
-	if (copy_from_user(&args, uargs, size))
-		return -EFAULT;
+	err = copy_struct_from_user(&args, sizeof(args), uargs, usize);
+	if (err)
+		return err;
 
 	/*
 	 * Verify that higher 32bits of exit_signal are unset and that
-- 
2.23.0

[PATCH RESEND v3 3/4] sched_setattr: switch to copy_struct_from_user()

[Aleksa Sarai]

From: Aleksa Sarai <cyphar@cyphar.com>

The change is very straightforward, and helps unify the syscall
interface for struct-from-userspace syscalls. Ideally we could also
unify sched_getattr(2)-style syscalls as well, but unfortunately the
correct semantics for such syscalls are much less clear (see [1] for
more detail). In future we could come up with a more sane idea for how
the syscall interface should look.

[1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
     robustify sched_read_attr() ABI logic and code")

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 kernel/sched/core.c | 43 +++++++------------------------------------
 1 file changed, 7 insertions(+), 36 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7880f4f64d0e..dd05a378631a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5106,9 +5106,6 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
 	u32 size;
 	int ret;
 
-	if (!access_ok(uattr, SCHED_ATTR_SIZE_VER0))
-		return -EFAULT;
-
 	/* Zero the full structure, so that a short copy will be nice: */
 	memset(attr, 0, sizeof(*attr));
 
@@ -5116,45 +5113,19 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
 	if (ret)
 		return ret;
 
-	/* Bail out on silly large: */
-	if (size > PAGE_SIZE)
-		goto err_size;
-
 	/* ABI compatibility quirk: */
 	if (!size)
 		size = SCHED_ATTR_SIZE_VER0;
-
-	if (size < SCHED_ATTR_SIZE_VER0)
+	if (size < SCHED_ATTR_SIZE_VER0 || size > PAGE_SIZE)
 		goto err_size;
 
-	/*
-	 * If we're handed a bigger struct than we know of,
-	 * ensure all the unknown bits are 0 - i.e. new
-	 * user-space does not rely on any kernel feature
-	 * extensions we dont know about yet.
-	 */
-	if (size > sizeof(*attr)) {
-		unsigned char __user *addr;
-		unsigned char __user *end;
-		unsigned char val;
-
-		addr = (void __user *)uattr + sizeof(*attr);
-		end  = (void __user *)uattr + size;
-
-		for (; addr < end; addr++) {
-			ret = get_user(val, addr);
-			if (ret)
-				return ret;
-			if (val)
-				goto err_size;
-		}
-		size = sizeof(*attr);
+	ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
+	if (ret) {
+		if (ret == -E2BIG)
+			goto err_size;
+		return ret;
 	}
 
-	ret = copy_from_user(attr, uattr, size);
-	if (ret)
-		return -EFAULT;
-
 	if ((attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) &&
 	    size < SCHED_ATTR_SIZE_VER1)
 		return -EINVAL;
@@ -5354,7 +5325,7 @@ sched_attr_copy_to_user(struct sched_attr __user *uattr,
  * sys_sched_getattr - similar to sched_getparam, but with sched_attr
  * @pid: the pid in question.
  * @uattr: structure containing the extended parameters.
- * @usize: sizeof(attr) that user-space knows about, for forwards and backwards compatibility.
+ * @usize: sizeof(attr) for fwd/bwd comp.
  * @flags: for future extension.
  */
 SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
-- 
2.23.0

[PATCH RESEND v3 4/4] perf_event_open: switch to copy_struct_from_user()

[Aleksa Sarai]

From: Aleksa Sarai <cyphar@cyphar.com>

The change is very straightforward, and helps unify the syscall
interface for struct-from-userspace syscalls.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
 kernel/events/core.c | 47 +++++++++-----------------------------------
 1 file changed, 9 insertions(+), 38 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4655adbbae10..3f0cb82e4fbc 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10586,55 +10586,26 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
 	u32 size;
 	int ret;
 
-	if (!access_ok(uattr, PERF_ATTR_SIZE_VER0))
-		return -EFAULT;
-
-	/*
-	 * zero the full structure, so that a short copy will be nice.
-	 */
+	/* Zero the full structure, so that a short copy will be nice. */
 	memset(attr, 0, sizeof(*attr));
 
 	ret = get_user(size, &uattr->size);
 	if (ret)
 		return ret;
 
-	if (size > PAGE_SIZE)	/* silly large */
-		goto err_size;
-
-	if (!size)		/* abi compat */
+	/* ABI compatibility quirk: */
+	if (!size)
 		size = PERF_ATTR_SIZE_VER0;
-
-	if (size < PERF_ATTR_SIZE_VER0)
+	if (size < PERF_ATTR_SIZE_VER0 || size > PAGE_SIZE)
 		goto err_size;
 
-	/*
-	 * If we're handed a bigger struct than we know of,
-	 * ensure all the unknown bits are 0 - i.e. new
-	 * user-space does not rely on any kernel feature
-	 * extensions we dont know about yet.
-	 */
-	if (size > sizeof(*attr)) {
-		unsigned char __user *addr;
-		unsigned char __user *end;
-		unsigned char val;
-
-		addr = (void __user *)uattr + sizeof(*attr);
-		end  = (void __user *)uattr + size;
-
-		for (; addr < end; addr++) {
-			ret = get_user(val, addr);
-			if (ret)
-				return ret;
-			if (val)
-				goto err_size;
-		}
-		size = sizeof(*attr);
+	ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
+	if (ret) {
+		if (ret == -E2BIG)
+			goto err_size;
+		return ret;
 	}
 
-	ret = copy_from_user(attr, uattr, size);
-	if (ret)
-		return -EFAULT;
-
 	attr->size = size;
 
 	if (attr->__reserved_1)
-- 
2.23.0

Re: [PATCH RESEND v3 1/4] lib: introduce copy_struct_from_user() helper

[Kees Cook]

On Tue, Oct 01, 2019 at 05:15:23AM +1000, Aleksa Sarai wrote:
> From: Aleksa Sarai <cyphar@cyphar.com>
> 
> A common pattern for syscall extensions is increasing the size of a
> struct passed from userspace, such that the zero-value of the new fields
> result in the old kernel behaviour (allowing for a mix of userspace and
> kernel vintages to operate on one another in most cases).
> 
> While this interface exists for communication in both directions, only
> one interface is straightforward to have reasonable semantics for
> (userspace passing a struct to the kernel). For kernel returns to
> userspace, what the correct semantics are (whether there should be an
> error if userspace is unaware of a new extension) is very
> syscall-dependent and thus probably cannot be unified between syscalls
> (a good example of this problem is [1]).
> 
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[2]). Future
> patches replace common uses of this pattern to make use of
> copy_struct_from_user().
> 
> Some in-kernel selftests that insure that the handling of alignment and
> various byte patterns are all handled identically to memchr_inv() usage.
> 
> [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
>      robustify sched_read_attr() ABI logic and code")
> 
> [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
>      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
>      always rejects differently-sized struct arguments.
> 
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/linux/bitops.h  |   7 +++
>  include/linux/uaccess.h |   4 ++
>  lib/strnlen_user.c      |   8 +--
>  lib/test_user_copy.c    | 133 ++++++++++++++++++++++++++++++++++++++--
>  lib/usercopy.c          | 123 +++++++++++++++++++++++++++++++++++++
>  5 files changed, 262 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> index cf074bce3eb3..c94a9ff9f082 100644
> --- a/include/linux/bitops.h
> +++ b/include/linux/bitops.h
> @@ -4,6 +4,13 @@
>  #include <asm/types.h>
>  #include <linux/bits.h>
>  
> +/* Set bits in the first 'n' bytes when loaded from memory */
> +#ifdef __LITTLE_ENDIAN
> +#  define aligned_byte_mask(n) ((1UL << 8*(n))-1)
> +#else
> +#  define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
> +#endif
> +
>  #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
>  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>  
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 70bbdc38dc37..94f20e6ec6ab 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -231,6 +231,10 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
>  
>  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
>  
> +extern int check_zeroed_user(const void __user *from, size_t size);
> +extern int copy_struct_from_user(void *dst, size_t ksize,
> +				 const void __user *src, size_t usize);
> +
>  /*
>   * probe_kernel_read(): safely attempt to read from a location
>   * @dst: pointer to the buffer that shall take the data
> diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> index 28ff554a1be8..6c0005d5dd5c 100644
> --- a/lib/strnlen_user.c
> +++ b/lib/strnlen_user.c
> @@ -3,16 +3,10 @@
>  #include <linux/export.h>
>  #include <linux/uaccess.h>
>  #include <linux/mm.h>
> +#include <linux/bitops.h>
>  
>  #include <asm/word-at-a-time.h>
>  
> -/* Set bits in the first 'n' bytes when loaded from memory */
> -#ifdef __LITTLE_ENDIAN
> -#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> -#else
> -#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> -#endif
> -
>  /*
>   * Do a strnlen, return length of string *with* final '\0'.
>   * 'count' is the user-supplied count, while 'max' is the
> diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> index 67bcd5dfd847..3a17f71029bb 100644
> --- a/lib/test_user_copy.c
> +++ b/lib/test_user_copy.c
> @@ -16,6 +16,7 @@
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
>  #include <linux/vmalloc.h>
> +#include <linux/random.h>
>  
>  /*
>   * Several 32-bit architectures support 64-bit {get,put}_user() calls.
> @@ -31,14 +32,129 @@
>  # define TEST_U64
>  #endif
>  
> -#define test(condition, msg)		\
> -({					\
> -	int cond = (condition);		\
> -	if (cond)			\
> -		pr_warn("%s\n", msg);	\
> -	cond;				\
> +#define test(condition, msg, ...)					\
> +({									\
> +	int cond = (condition);						\
> +	if (cond)							\
> +		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
> +	cond;								\
>  })
>  
> +static bool is_zeroed(void *from, size_t size)
> +{
> +	return memchr_inv(from, 0x0, size) == NULL;
> +}
> +
> +static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
> +{
> +	int ret = 0;
> +	size_t start, end, i;
> +	size_t zero_start = size / 4;
> +	size_t zero_end = size - zero_start;
> +
> +	/*
> +	 * We conduct a series of check_nonzero_user() tests on a block of memory
> +	 * with the following byte-pattern (trying every possible [start,end]
> +	 * pair):
> +	 *
> +	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> +	 *
> +	 * And we verify that check_nonzero_user() acts identically to memchr_inv().
> +	 */
> +
> +	memset(kmem, 0x0, size);
> +	for (i = 1; i < zero_start; i += 2)
> +		kmem[i] = 0xff;
> +	for (i = zero_end; i < size; i += 2)
> +		kmem[i] = 0xff;
> +
> +	ret |= test(copy_to_user(umem, kmem, size),
> +		    "legitimate copy_to_user failed");
> +
> +	for (start = 0; start <= size; start++) {
> +		for (end = start; end <= size; end++) {
> +			size_t len = end - start;
> +			int retval = check_zeroed_user(umem + start, len);
> +			int expected = is_zeroed(kmem + start, len);
> +
> +			ret |= test(retval != expected,
> +				    "check_nonzero_user(=%d) != memchr_inv(=%d) mismatch (start=%zu, end=%zu)",
> +				    retval, expected, start, end);
> +		}
> +	}
> +
> +	return ret;
> +}
> +
> +static int test_copy_struct_from_user(char *kmem, char __user *umem,
> +				      size_t size)
> +{
> +	int ret = 0;
> +	char *rand = NULL, *expected = NULL;
> +	size_t ksize, usize;
> +
> +	rand = kmalloc(size, GFP_KERNEL);
> +	if (ret |= test(rand == NULL, "kmalloc failed"))
> +		goto out_free;
> +
> +	expected = kmalloc(size, GFP_KERNEL);
> +	if (ret |= test(expected == NULL, "kmalloc failed"))
> +		goto out_free;
> +
> +	/* Fill umem with random bytes. */
> +	memset(kmem, 0x0, size);
> +	prandom_bytes(rand, size);

I don't really like using random() in tests on the chance that we get
failures we can't reproduced. If you want to do this (instead of using a
byte-fill pattern), you need to dump the entire state of the memory
region. Why not just memset(rand, 0xff, ...)? (And obviously rename
"rand")

> +	ret |= test(copy_to_user(umem, rand, size),
> +		    "legitimate copy_to_user failed");
> +
> +	/* Check basic case -- (usize == ksize). */
> +	ksize = size;
> +	usize = size;

I'd move the memset(kmem, 0x0, size); down to here.

> +	memcpy(expected, rand, ksize);
> +
> +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> +		    "copy_struct_from_user(usize == ksize) failed");
> +	ret |= test(memcmp(kmem, expected, ksize),
> +		    "copy_struct_from_user(usize == ksize) gives unexpected copy");
> +
> +	/* Old userspace case -- (usize < ksize). */
> +	ksize = size;
> +	usize = ksize / 2;
> +

I would expect memset(kmem, 0x0, size); again here since a new test of
that region is starting.

> +	memcpy(expected, rand, usize);
> +	memset(expected + usize, 0x0, ksize - usize);
> +
> +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> +		    "copy_struct_from_user(usize < ksize) failed");
> +	ret |= test(memcmp(kmem, expected, ksize),
> +		    "copy_struct_from_user(usize < ksize) gives unexpected copy");
> +
> +	/* New userspace (-E2BIG) case -- (usize > ksize). */
> +	usize = size;
> +	ksize = usize / 2;

and here?

> +
> +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize) != -E2BIG,
> +		    "copy_struct_from_user(usize > ksize) didn't give E2BIG");
> +
> +	/* New userspace (success) case -- (usize > ksize). */
> +	usize = size;
> +	ksize = usize / 2;
> +

and here?

> +	memcpy(expected, rand, ksize);
> +
> +	ret |= test(clear_user(umem + ksize, usize - ksize),
> +		    "legitimate clear_user failed");
> +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> +		    "copy_struct_from_user(usize > ksize) failed");
> +	ret |= test(memcmp(kmem, expected, ksize),
> +		    "copy_struct_from_user(usize > ksize) gives unexpected copy");
> +
> +out_free:
> +	kfree(expected);
> +	kfree(rand);
> +	return ret;
> +}
> +
>  static int __init test_user_copy_init(void)
>  {
>  	int ret = 0;
> @@ -106,6 +222,11 @@ static int __init test_user_copy_init(void)
>  #endif
>  #undef test_legit
>  
> +	/* Test usage of check_nonzero_user(). */
> +	ret |= test_check_nonzero_user(kmem, usermem, 2 * PAGE_SIZE);
> +	/* Test usage of copy_struct_from_user(). */
> +	ret |= test_copy_struct_from_user(kmem, usermem, 2 * PAGE_SIZE);
> +
>  	/*
>  	 * Invalid usage: none of these copies should succeed.
>  	 */
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index c2bfbcaeb3dc..cf7f854ed9c8 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/uaccess.h>
> +#include <linux/bitops.h>
>  
>  /* out-of-line parts */
>  
> @@ -31,3 +32,125 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
>  }
>  EXPORT_SYMBOL(_copy_to_user);
>  #endif
> +
> +/**
> + * check_zeroed_user: check if a userspace buffer only contains zero bytes
> + * @from: Source address, in userspace.
> + * @size: Size of buffer.
> + *
> + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
> + * userspace addresses (and is more efficient because we don't care where the
> + * first non-zero byte is).
> + *
> + * Returns:
> + *  * 0: There were non-zero bytes present in the buffer.
> + *  * 1: The buffer was full of zero bytes.
> + *  * -EFAULT: access to userspace failed.
> + */
> +int check_zeroed_user(const void __user *from, size_t size)
> +{
> +	unsigned long val;
> +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> +
> +	if (unlikely(size == 0))
> +		return 1;
> +
> +	from -= align;
> +	size += align;
> +
> +	if (!user_access_begin(from, size))
> +		return -EFAULT;
> +
> +	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	if (align)
> +		val &= ~aligned_byte_mask(align);
> +
> +	while (size > sizeof(unsigned long)) {
> +		if (unlikely(val))
> +			goto done;
> +
> +		from += sizeof(unsigned long);
> +		size -= sizeof(unsigned long);
> +
> +		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> +	}
> +
> +	if (size < sizeof(unsigned long))
> +		val &= aligned_byte_mask(size);
> +
> +done:
> +	user_access_end();
> +	return (val == 0);
> +err_fault:
> +	user_access_end();
> +	return -EFAULT;
> +}
> +EXPORT_SYMBOL(check_zeroed_user);
> +
> +/**
> + * copy_struct_from_user: copy a struct from userspace
> + * @dst:   Destination address, in kernel space. This buffer must be @ksize
> + *         bytes long.
> + * @ksize: Size of @dst struct.
> + * @src:   Source address, in userspace.
> + * @usize: (Alleged) size of @src struct.
> + *
> + * Copies a struct from userspace to kernel space, in a way that guarantees
> + * backwards-compatibility for struct syscall arguments (as long as future
> + * struct extensions are made such that all new fields are *appended* to the
> + * old struct, and zeroed-out new fields have the same meaning as the old
> + * struct).
> + *
> + * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
> + * The recommended usage is something like the following:
> + *
> + *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
> + *   {
> + *      int err;
> + *      struct foo karg = {};
> + *
> + *      if (usize > PAGE_SIZE)
> + *        return -E2BIG;
> + *      if (usize < FOO_SIZE_VER0)
> + *        return -EINVAL;
> + *
> + *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, usize);
> + *      if (err)
> + *        return err;
> + *
> + *      // ...
> + *   }
> + *
> + * There are three cases to consider:
> + *  * If @usize == @ksize, then it's copied verbatim.
> + *  * If @usize < @ksize, then the userspace has passed an old struct to a
> + *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
> + *    are to be zero-filled.
> + *  * If @usize > @ksize, then the userspace has passed a new struct to an
> + *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
> + *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
> + *
> + * Returns (in all cases, some data may have been copied):
> + *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
> + *  * -EFAULT: access to userspace failed.
> + */
> +int copy_struct_from_user(void *dst, size_t ksize,
> +			  const void __user *src, size_t usize)

I'd like to see this be marked __always_inline so the dst type is known
to the compiler during the copy_from_user() size sanity checks, etc.

> +{
> +	size_t size = min(ksize, usize);
> +	size_t rest = max(ksize, usize) - size;
> +
> +	/* Deal with trailing bytes. */
> +	if (usize < ksize) {
> +		memset(dst + size, 0, rest);
> +	} else if (usize > ksize) {
> +		int ret = check_zeroed_user(src + size, rest);
> +		if (ret <= 0)
> +			return ret ?: -E2BIG;
> +	}
> +	/* Copy the interoperable parts of the struct. */
> +	if (copy_from_user(dst, src, size))
> +		return -EFAULT;
> +	return 0;
> +}
> +EXPORT_SYMBOL(copy_struct_from_user);
> -- 
> 2.23.0
> 

But besides those things, yes please. :)

-- 
Kees Cook

Re: [PATCH RESEND v3 2/4] clone3: switch to copy_struct_from_user()

[Kees Cook]

On Tue, Oct 01, 2019 at 05:15:24AM +1000, Aleksa Sarai wrote:
> From: Aleksa Sarai <cyphar@cyphar.com>
> 
> The change is very straightforward, and helps unify the syscall
> interface for struct-from-userspace syscalls. Additionally, explicitly
> define CLONE_ARGS_SIZE_VER0 to match the other users of the
> struct-extension pattern.
> 
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> ---
>  include/uapi/linux/sched.h |  2 ++
>  kernel/fork.c              | 34 +++++++---------------------------
>  2 files changed, 9 insertions(+), 27 deletions(-)
> 
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index b3105ac1381a..0945805982b4 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -47,6 +47,8 @@ struct clone_args {
>  	__aligned_u64 tls;
>  };
>  
> +#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
> +
>  /*
>   * Scheduling policies
>   */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f9572f416126..2ef529869c64 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2525,39 +2525,19 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
>  #ifdef __ARCH_WANT_SYS_CLONE3
>  noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
>  					      struct clone_args __user *uargs,
> -					      size_t size)
> +					      size_t usize)
>  {
> +	int err;
>  	struct clone_args args;
>  
> -	if (unlikely(size > PAGE_SIZE))
> +	if (unlikely(usize > PAGE_SIZE))
>  		return -E2BIG;

I quickly looked through the earlier threads and couldn't find it, but
I have a memory of some discussion about moving this test into the
copy_struct_from_user() function itself? That would seems like a
reasonable idea? ("4k should be enough for any structure!")

Either way:

Reviewed-by: Kees Cook <keescook@chromium.org>


> -
> -	if (unlikely(size < sizeof(struct clone_args)))
> +	if (unlikely(usize < CLONE_ARGS_SIZE_VER0))
>  		return -EINVAL;
>  
> -	if (unlikely(!access_ok(uargs, size)))
> -		return -EFAULT;
> -
> -	if (size > sizeof(struct clone_args)) {
> -		unsigned char __user *addr;
> -		unsigned char __user *end;
> -		unsigned char val;
> -
> -		addr = (void __user *)uargs + sizeof(struct clone_args);
> -		end = (void __user *)uargs + size;
> -
> -		for (; addr < end; addr++) {
> -			if (get_user(val, addr))
> -				return -EFAULT;
> -			if (val)
> -				return -E2BIG;
> -		}
> -
> -		size = sizeof(struct clone_args);
> -	}
> -
> -	if (copy_from_user(&args, uargs, size))
> -		return -EFAULT;
> +	err = copy_struct_from_user(&args, sizeof(args), uargs, usize);
> +	if (err)
> +		return err;
>  
>  	/*
>  	 * Verify that higher 32bits of exit_signal are unset and that
> -- 
> 2.23.0
> 

-- 
Kees Cook

Re: [PATCH RESEND v3 3/4] sched_setattr: switch to copy_struct_from_user()

[Kees Cook]

On Tue, Oct 01, 2019 at 05:15:25AM +1000, Aleksa Sarai wrote:
> From: Aleksa Sarai <cyphar@cyphar.com>
> 
> The change is very straightforward, and helps unify the syscall
> interface for struct-from-userspace syscalls. Ideally we could also
> unify sched_getattr(2)-style syscalls as well, but unfortunately the
> correct semantics for such syscalls are much less clear (see [1] for
> more detail). In future we could come up with a more sane idea for how
> the syscall interface should look.
> 
> [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
>      robustify sched_read_attr() ABI logic and code")
> 
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  kernel/sched/core.c | 43 +++++++------------------------------------
>  1 file changed, 7 insertions(+), 36 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7880f4f64d0e..dd05a378631a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -5106,9 +5106,6 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
>  	u32 size;
>  	int ret;
>  
> -	if (!access_ok(uattr, SCHED_ATTR_SIZE_VER0))
> -		return -EFAULT;
> -
>  	/* Zero the full structure, so that a short copy will be nice: */
>  	memset(attr, 0, sizeof(*attr));
>  
> @@ -5116,45 +5113,19 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
>  	if (ret)
>  		return ret;
>  
> -	/* Bail out on silly large: */
> -	if (size > PAGE_SIZE)
> -		goto err_size;
> -
>  	/* ABI compatibility quirk: */
>  	if (!size)
>  		size = SCHED_ATTR_SIZE_VER0;
> -
> -	if (size < SCHED_ATTR_SIZE_VER0)
> +	if (size < SCHED_ATTR_SIZE_VER0 || size > PAGE_SIZE)
>  		goto err_size;
>  
> -	/*
> -	 * If we're handed a bigger struct than we know of,
> -	 * ensure all the unknown bits are 0 - i.e. new
> -	 * user-space does not rely on any kernel feature
> -	 * extensions we dont know about yet.
> -	 */
> -	if (size > sizeof(*attr)) {
> -		unsigned char __user *addr;
> -		unsigned char __user *end;
> -		unsigned char val;
> -
> -		addr = (void __user *)uattr + sizeof(*attr);
> -		end  = (void __user *)uattr + size;
> -
> -		for (; addr < end; addr++) {
> -			ret = get_user(val, addr);
> -			if (ret)
> -				return ret;
> -			if (val)
> -				goto err_size;
> -		}
> -		size = sizeof(*attr);
> +	ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
> +	if (ret) {
> +		if (ret == -E2BIG)
> +			goto err_size;
> +		return ret;
>  	}
>  
> -	ret = copy_from_user(attr, uattr, size);
> -	if (ret)
> -		return -EFAULT;
> -
>  	if ((attr->sched_flags & SCHED_FLAG_UTIL_CLAMP) &&
>  	    size < SCHED_ATTR_SIZE_VER1)
>  		return -EINVAL;
> @@ -5354,7 +5325,7 @@ sched_attr_copy_to_user(struct sched_attr __user *uattr,
>   * sys_sched_getattr - similar to sched_getparam, but with sched_attr
>   * @pid: the pid in question.
>   * @uattr: structure containing the extended parameters.
> - * @usize: sizeof(attr) that user-space knows about, for forwards and backwards compatibility.
> + * @usize: sizeof(attr) for fwd/bwd comp.
>   * @flags: for future extension.
>   */
>  SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> -- 
> 2.23.0
> 

-- 
Kees Cook

Re: [PATCH RESEND v3 4/4] perf_event_open: switch to copy_struct_from_user()

[Kees Cook]

On Tue, Oct 01, 2019 at 05:15:26AM +1000, Aleksa Sarai wrote:
> From: Aleksa Sarai <cyphar@cyphar.com>
> 
> The change is very straightforward, and helps unify the syscall
> interface for struct-from-userspace syscalls.
> 
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>  kernel/events/core.c | 47 +++++++++-----------------------------------
>  1 file changed, 9 insertions(+), 38 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4655adbbae10..3f0cb82e4fbc 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -10586,55 +10586,26 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
>  	u32 size;
>  	int ret;
>  
> -	if (!access_ok(uattr, PERF_ATTR_SIZE_VER0))
> -		return -EFAULT;
> -
> -	/*
> -	 * zero the full structure, so that a short copy will be nice.
> -	 */
> +	/* Zero the full structure, so that a short copy will be nice. */
>  	memset(attr, 0, sizeof(*attr));
>  
>  	ret = get_user(size, &uattr->size);
>  	if (ret)
>  		return ret;
>  
> -	if (size > PAGE_SIZE)	/* silly large */
> -		goto err_size;
> -
> -	if (!size)		/* abi compat */
> +	/* ABI compatibility quirk: */
> +	if (!size)
>  		size = PERF_ATTR_SIZE_VER0;
> -
> -	if (size < PERF_ATTR_SIZE_VER0)
> +	if (size < PERF_ATTR_SIZE_VER0 || size > PAGE_SIZE)
>  		goto err_size;
>  
> -	/*
> -	 * If we're handed a bigger struct than we know of,
> -	 * ensure all the unknown bits are 0 - i.e. new
> -	 * user-space does not rely on any kernel feature
> -	 * extensions we dont know about yet.
> -	 */
> -	if (size > sizeof(*attr)) {
> -		unsigned char __user *addr;
> -		unsigned char __user *end;
> -		unsigned char val;
> -
> -		addr = (void __user *)uattr + sizeof(*attr);
> -		end  = (void __user *)uattr + size;
> -
> -		for (; addr < end; addr++) {
> -			ret = get_user(val, addr);
> -			if (ret)
> -				return ret;
> -			if (val)
> -				goto err_size;
> -		}
> -		size = sizeof(*attr);
> +	ret = copy_struct_from_user(attr, sizeof(*attr), uattr, size);
> +	if (ret) {
> +		if (ret == -E2BIG)
> +			goto err_size;
> +		return ret;
>  	}
>  
> -	ret = copy_from_user(attr, uattr, size);
> -	if (ret)
> -		return -EFAULT;
> -
>  	attr->size = size;
>  
>  	if (attr->__reserved_1)
> -- 
> 2.23.0
> 

-- 
Kees Cook

Re: [PATCH RESEND v3 1/4] lib: introduce copy_struct_from_user() helper

[Aleksa Sarai]

[-- Attachment #1: Type: text/plain, Size: 15430 bytes --]

On 2019-09-30, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Oct 01, 2019 at 05:15:23AM +1000, Aleksa Sarai wrote:
> > From: Aleksa Sarai <cyphar@cyphar.com>
> > 
> > A common pattern for syscall extensions is increasing the size of a
> > struct passed from userspace, such that the zero-value of the new fields
> > result in the old kernel behaviour (allowing for a mix of userspace and
> > kernel vintages to operate on one another in most cases).
> > 
> > While this interface exists for communication in both directions, only
> > one interface is straightforward to have reasonable semantics for
> > (userspace passing a struct to the kernel). For kernel returns to
> > userspace, what the correct semantics are (whether there should be an
> > error if userspace is unaware of a new extension) is very
> > syscall-dependent and thus probably cannot be unified between syscalls
> > (a good example of this problem is [1]).
> > 
> > Previously there was no common lib/ function that implemented
> > the necessary extension-checking semantics (and different syscalls
> > implemented them slightly differently or incompletely[2]). Future
> > patches replace common uses of this pattern to make use of
> > copy_struct_from_user().
> > 
> > Some in-kernel selftests that insure that the handling of alignment and
> > various byte patterns are all handled identically to memchr_inv() usage.
> > 
> > [1]: commit 1251201c0d34 ("sched/core: Fix uclamp ABI bug, clean up and
> >      robustify sched_read_attr() ABI logic and code")
> > 
> > [2]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
> >      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
> >      always rejects differently-sized struct arguments.
> > 
> > Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > ---
> >  include/linux/bitops.h  |   7 +++
> >  include/linux/uaccess.h |   4 ++
> >  lib/strnlen_user.c      |   8 +--
> >  lib/test_user_copy.c    | 133 ++++++++++++++++++++++++++++++++++++++--
> >  lib/usercopy.c          | 123 +++++++++++++++++++++++++++++++++++++
> >  5 files changed, 262 insertions(+), 13 deletions(-)
> > 
> > diff --git a/include/linux/bitops.h b/include/linux/bitops.h
> > index cf074bce3eb3..c94a9ff9f082 100644
> > --- a/include/linux/bitops.h
> > +++ b/include/linux/bitops.h
> > @@ -4,6 +4,13 @@
> >  #include <asm/types.h>
> >  #include <linux/bits.h>
> >  
> > +/* Set bits in the first 'n' bytes when loaded from memory */
> > +#ifdef __LITTLE_ENDIAN
> > +#  define aligned_byte_mask(n) ((1UL << 8*(n))-1)
> > +#else
> > +#  define aligned_byte_mask(n) (~0xffUL << (BITS_PER_LONG - 8 - 8*(n)))
> > +#endif
> > +
> >  #define BITS_PER_TYPE(type) (sizeof(type) * BITS_PER_BYTE)
> >  #define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
> >  
> > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> > index 70bbdc38dc37..94f20e6ec6ab 100644
> > --- a/include/linux/uaccess.h
> > +++ b/include/linux/uaccess.h
> > @@ -231,6 +231,10 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
> >  
> >  #endif		/* ARCH_HAS_NOCACHE_UACCESS */
> >  
> > +extern int check_zeroed_user(const void __user *from, size_t size);
> > +extern int copy_struct_from_user(void *dst, size_t ksize,
> > +				 const void __user *src, size_t usize);
> > +
> >  /*
> >   * probe_kernel_read(): safely attempt to read from a location
> >   * @dst: pointer to the buffer that shall take the data
> > diff --git a/lib/strnlen_user.c b/lib/strnlen_user.c
> > index 28ff554a1be8..6c0005d5dd5c 100644
> > --- a/lib/strnlen_user.c
> > +++ b/lib/strnlen_user.c
> > @@ -3,16 +3,10 @@
> >  #include <linux/export.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/mm.h>
> > +#include <linux/bitops.h>
> >  
> >  #include <asm/word-at-a-time.h>
> >  
> > -/* Set bits in the first 'n' bytes when loaded from memory */
> > -#ifdef __LITTLE_ENDIAN
> > -#  define aligned_byte_mask(n) ((1ul << 8*(n))-1)
> > -#else
> > -#  define aligned_byte_mask(n) (~0xfful << (BITS_PER_LONG - 8 - 8*(n)))
> > -#endif
> > -
> >  /*
> >   * Do a strnlen, return length of string *with* final '\0'.
> >   * 'count' is the user-supplied count, while 'max' is the
> > diff --git a/lib/test_user_copy.c b/lib/test_user_copy.c
> > index 67bcd5dfd847..3a17f71029bb 100644
> > --- a/lib/test_user_copy.c
> > +++ b/lib/test_user_copy.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/slab.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/vmalloc.h>
> > +#include <linux/random.h>
> >  
> >  /*
> >   * Several 32-bit architectures support 64-bit {get,put}_user() calls.
> > @@ -31,14 +32,129 @@
> >  # define TEST_U64
> >  #endif
> >  
> > -#define test(condition, msg)		\
> > -({					\
> > -	int cond = (condition);		\
> > -	if (cond)			\
> > -		pr_warn("%s\n", msg);	\
> > -	cond;				\
> > +#define test(condition, msg, ...)					\
> > +({									\
> > +	int cond = (condition);						\
> > +	if (cond)							\
> > +		pr_warn("[%d] " msg "\n", __LINE__, ##__VA_ARGS__);	\
> > +	cond;								\
> >  })
> >  
> > +static bool is_zeroed(void *from, size_t size)
> > +{
> > +	return memchr_inv(from, 0x0, size) == NULL;
> > +}
> > +
> > +static int test_check_nonzero_user(char *kmem, char __user *umem, size_t size)
> > +{
> > +	int ret = 0;
> > +	size_t start, end, i;
> > +	size_t zero_start = size / 4;
> > +	size_t zero_end = size - zero_start;
> > +
> > +	/*
> > +	 * We conduct a series of check_nonzero_user() tests on a block of memory
> > +	 * with the following byte-pattern (trying every possible [start,end]
> > +	 * pair):
> > +	 *
> > +	 *   [ 00 ff 00 ff ... 00 00 00 00 ... ff 00 ff 00 ]
> > +	 *
> > +	 * And we verify that check_nonzero_user() acts identically to memchr_inv().
> > +	 */
> > +
> > +	memset(kmem, 0x0, size);
> > +	for (i = 1; i < zero_start; i += 2)
> > +		kmem[i] = 0xff;
> > +	for (i = zero_end; i < size; i += 2)
> > +		kmem[i] = 0xff;
> > +
> > +	ret |= test(copy_to_user(umem, kmem, size),
> > +		    "legitimate copy_to_user failed");
> > +
> > +	for (start = 0; start <= size; start++) {
> > +		for (end = start; end <= size; end++) {
> > +			size_t len = end - start;
> > +			int retval = check_zeroed_user(umem + start, len);
> > +			int expected = is_zeroed(kmem + start, len);
> > +
> > +			ret |= test(retval != expected,
> > +				    "check_nonzero_user(=%d) != memchr_inv(=%d) mismatch (start=%zu, end=%zu)",
> > +				    retval, expected, start, end);
> > +		}
> > +	}
> > +
> > +	return ret;
> > +}
> > +
> > +static int test_copy_struct_from_user(char *kmem, char __user *umem,
> > +				      size_t size)
> > +{
> > +	int ret = 0;
> > +	char *rand = NULL, *expected = NULL;
> > +	size_t ksize, usize;
> > +
> > +	rand = kmalloc(size, GFP_KERNEL);
> > +	if (ret |= test(rand == NULL, "kmalloc failed"))
> > +		goto out_free;
> > +
> > +	expected = kmalloc(size, GFP_KERNEL);
> > +	if (ret |= test(expected == NULL, "kmalloc failed"))
> > +		goto out_free;
> > +
> > +	/* Fill umem with random bytes. */
> > +	memset(kmem, 0x0, size);
> > +	prandom_bytes(rand, size);
> 
> I don't really like using random() in tests on the chance that we get
> failures we can't reproduced. If you want to do this (instead of using a
> byte-fill pattern), you need to dump the entire state of the memory
> region. Why not just memset(rand, 0xff, ...)? (And obviously rename
> "rand")

Fair enough.

> > +	ret |= test(copy_to_user(umem, rand, size),
> > +		    "legitimate copy_to_user failed");
> > +
> > +	/* Check basic case -- (usize == ksize). */
> > +	ksize = size;
> > +	usize = size;
> 
> I'd move the memset(kmem, 0x0, size); down to here.
> 
> > +	memcpy(expected, rand, ksize);
> > +
> > +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> > +		    "copy_struct_from_user(usize == ksize) failed");
> > +	ret |= test(memcmp(kmem, expected, ksize),
> > +		    "copy_struct_from_user(usize == ksize) gives unexpected copy");
> > +
> > +	/* Old userspace case -- (usize < ksize). */
> > +	ksize = size;
> > +	usize = ksize / 2;
> > +
> 
> I would expect memset(kmem, 0x0, size); again here since a new test of
> that region is starting.
> 
> > +	memcpy(expected, rand, usize);
> > +	memset(expected + usize, 0x0, ksize - usize);
> > +
> > +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> > +		    "copy_struct_from_user(usize < ksize) failed");
> > +	ret |= test(memcmp(kmem, expected, ksize),
> > +		    "copy_struct_from_user(usize < ksize) gives unexpected copy");
> > +
> > +	/* New userspace (-E2BIG) case -- (usize > ksize). */
> > +	usize = size;
> > +	ksize = usize / 2;
> 
> and here?
> 
> > +
> > +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize) != -E2BIG,
> > +		    "copy_struct_from_user(usize > ksize) didn't give E2BIG");
> > +
> > +	/* New userspace (success) case -- (usize > ksize). */
> > +	usize = size;
> > +	ksize = usize / 2;
> > +
> 
> and here?

Will do all of the above.

> > +	memcpy(expected, rand, ksize);
> > +
> > +	ret |= test(clear_user(umem + ksize, usize - ksize),
> > +		    "legitimate clear_user failed");
> > +	ret |= test(copy_struct_from_user(kmem, ksize, umem, usize),
> > +		    "copy_struct_from_user(usize > ksize) failed");
> > +	ret |= test(memcmp(kmem, expected, ksize),
> > +		    "copy_struct_from_user(usize > ksize) gives unexpected copy");
> > +
> > +out_free:
> > +	kfree(expected);
> > +	kfree(rand);
> > +	return ret;
> > +}
> > +
> >  static int __init test_user_copy_init(void)
> >  {
> >  	int ret = 0;
> > @@ -106,6 +222,11 @@ static int __init test_user_copy_init(void)
> >  #endif
> >  #undef test_legit
> >  
> > +	/* Test usage of check_nonzero_user(). */
> > +	ret |= test_check_nonzero_user(kmem, usermem, 2 * PAGE_SIZE);
> > +	/* Test usage of copy_struct_from_user(). */
> > +	ret |= test_copy_struct_from_user(kmem, usermem, 2 * PAGE_SIZE);
> > +
> >  	/*
> >  	 * Invalid usage: none of these copies should succeed.
> >  	 */
> > diff --git a/lib/usercopy.c b/lib/usercopy.c
> > index c2bfbcaeb3dc..cf7f854ed9c8 100644
> > --- a/lib/usercopy.c
> > +++ b/lib/usercopy.c
> > @@ -1,5 +1,6 @@
> >  // SPDX-License-Identifier: GPL-2.0
> >  #include <linux/uaccess.h>
> > +#include <linux/bitops.h>
> >  
> >  /* out-of-line parts */
> >  
> > @@ -31,3 +32,125 @@ unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
> >  }
> >  EXPORT_SYMBOL(_copy_to_user);
> >  #endif
> > +
> > +/**
> > + * check_zeroed_user: check if a userspace buffer only contains zero bytes
> > + * @from: Source address, in userspace.
> > + * @size: Size of buffer.
> > + *
> > + * This is effectively shorthand for "memchr_inv(from, 0, size) == NULL" for
> > + * userspace addresses (and is more efficient because we don't care where the
> > + * first non-zero byte is).
> > + *
> > + * Returns:
> > + *  * 0: There were non-zero bytes present in the buffer.
> > + *  * 1: The buffer was full of zero bytes.
> > + *  * -EFAULT: access to userspace failed.
> > + */
> > +int check_zeroed_user(const void __user *from, size_t size)
> > +{
> > +	unsigned long val;
> > +	uintptr_t align = (uintptr_t) from % sizeof(unsigned long);
> > +
> > +	if (unlikely(size == 0))
> > +		return 1;
> > +
> > +	from -= align;
> > +	size += align;
> > +
> > +	if (!user_access_begin(from, size))
> > +		return -EFAULT;
> > +
> > +	unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> > +	if (align)
> > +		val &= ~aligned_byte_mask(align);
> > +
> > +	while (size > sizeof(unsigned long)) {
> > +		if (unlikely(val))
> > +			goto done;
> > +
> > +		from += sizeof(unsigned long);
> > +		size -= sizeof(unsigned long);
> > +
> > +		unsafe_get_user(val, (unsigned long __user *) from, err_fault);
> > +	}
> > +
> > +	if (size < sizeof(unsigned long))
> > +		val &= aligned_byte_mask(size);
> > +
> > +done:
> > +	user_access_end();
> > +	return (val == 0);
> > +err_fault:
> > +	user_access_end();
> > +	return -EFAULT;
> > +}
> > +EXPORT_SYMBOL(check_zeroed_user);
> > +
> > +/**
> > + * copy_struct_from_user: copy a struct from userspace
> > + * @dst:   Destination address, in kernel space. This buffer must be @ksize
> > + *         bytes long.
> > + * @ksize: Size of @dst struct.
> > + * @src:   Source address, in userspace.
> > + * @usize: (Alleged) size of @src struct.
> > + *
> > + * Copies a struct from userspace to kernel space, in a way that guarantees
> > + * backwards-compatibility for struct syscall arguments (as long as future
> > + * struct extensions are made such that all new fields are *appended* to the
> > + * old struct, and zeroed-out new fields have the same meaning as the old
> > + * struct).
> > + *
> > + * @ksize is just sizeof(*dst), and @usize should've been passed by userspace.
> > + * The recommended usage is something like the following:
> > + *
> > + *   SYSCALL_DEFINE2(foobar, const struct foo __user *, uarg, size_t, usize)
> > + *   {
> > + *      int err;
> > + *      struct foo karg = {};
> > + *
> > + *      if (usize > PAGE_SIZE)
> > + *        return -E2BIG;
> > + *      if (usize < FOO_SIZE_VER0)
> > + *        return -EINVAL;
> > + *
> > + *      err = copy_struct_from_user(&karg, sizeof(karg), uarg, usize);
> > + *      if (err)
> > + *        return err;
> > + *
> > + *      // ...
> > + *   }
> > + *
> > + * There are three cases to consider:
> > + *  * If @usize == @ksize, then it's copied verbatim.
> > + *  * If @usize < @ksize, then the userspace has passed an old struct to a
> > + *    newer kernel. The rest of the trailing bytes in @dst (@ksize - @usize)
> > + *    are to be zero-filled.
> > + *  * If @usize > @ksize, then the userspace has passed a new struct to an
> > + *    older kernel. The trailing bytes unknown to the kernel (@usize - @ksize)
> > + *    are checked to ensure they are zeroed, otherwise -E2BIG is returned.
> > + *
> > + * Returns (in all cases, some data may have been copied):
> > + *  * -E2BIG:  (@usize > @ksize) and there are non-zero trailing bytes in @src.
> > + *  * -EFAULT: access to userspace failed.
> > + */
> > +int copy_struct_from_user(void *dst, size_t ksize,
> > +			  const void __user *src, size_t usize)
> 
> I'd like to see this be marked __always_inline so the dst type is known
> to the compiler during the copy_from_user() size sanity checks, etc.

Sure, will do. I'll put it in uaccess.h.

> > +{
> > +	size_t size = min(ksize, usize);
> > +	size_t rest = max(ksize, usize) - size;
> > +
> > +	/* Deal with trailing bytes. */
> > +	if (usize < ksize) {
> > +		memset(dst + size, 0, rest);
> > +	} else if (usize > ksize) {
> > +		int ret = check_zeroed_user(src + size, rest);
> > +		if (ret <= 0)
> > +			return ret ?: -E2BIG;
> > +	}
> > +	/* Copy the interoperable parts of the struct. */
> > +	if (copy_from_user(dst, src, size))
> > +		return -EFAULT;
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(copy_struct_from_user);
> > -- 
> > 2.23.0

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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Re: [PATCH RESEND v3 2/4] clone3: switch to copy_struct_from_user()

[Aleksa Sarai]

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On 2019-09-30, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Oct 01, 2019 at 05:15:24AM +1000, Aleksa Sarai wrote:
> > From: Aleksa Sarai <cyphar@cyphar.com>
> > 
> > The change is very straightforward, and helps unify the syscall
> > interface for struct-from-userspace syscalls. Additionally, explicitly
> > define CLONE_ARGS_SIZE_VER0 to match the other users of the
> > struct-extension pattern.
> > 
> > Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
> > ---
> >  include/uapi/linux/sched.h |  2 ++
> >  kernel/fork.c              | 34 +++++++---------------------------
> >  2 files changed, 9 insertions(+), 27 deletions(-)
> > 
> > diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> > index b3105ac1381a..0945805982b4 100644
> > --- a/include/uapi/linux/sched.h
> > +++ b/include/uapi/linux/sched.h
> > @@ -47,6 +47,8 @@ struct clone_args {
> >  	__aligned_u64 tls;
> >  };
> >  
> > +#define CLONE_ARGS_SIZE_VER0 64 /* sizeof first published struct */
> > +
> >  /*
> >   * Scheduling policies
> >   */
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index f9572f416126..2ef529869c64 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -2525,39 +2525,19 @@ SYSCALL_DEFINE5(clone, unsigned long, clone_flags, unsigned long, newsp,
> >  #ifdef __ARCH_WANT_SYS_CLONE3
> >  noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
> >  					      struct clone_args __user *uargs,
> > -					      size_t size)
> > +					      size_t usize)
> >  {
> > +	int err;
> >  	struct clone_args args;
> >  
> > -	if (unlikely(size > PAGE_SIZE))
> > +	if (unlikely(usize > PAGE_SIZE))
> >  		return -E2BIG;
> 
> I quickly looked through the earlier threads and couldn't find it, but
> I have a memory of some discussion about moving this test into the
> copy_struct_from_user() function itself? That would seems like a
> reasonable idea? ("4k should be enough for any structure!")

Yes (and this also seemed the most reasonable way to do it to me), but
the main counter-arguments which swayed me were:

 1. Putting it in the hands of the caller allows them to decide if they
    want to have a limit, because if you institute a limit in one kernel
    vintage then expanding it later will be less-than-ideally-smooth.

 2. There is no amplification, so doing copy_struct_from_user() for a
    really big usize boils down to the userspace program blocking for
    the kernel to check if some of your memory is zeroed. Thus there
    doesn't seem to be much DoS potential.

Not to mention that users of copy_struct_from_user() will end up doing
some kind of usize comparison anyway (to check if it's smaller than
the version-0 size).

> Either way:
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> 
> 
> > -
> > -	if (unlikely(size < sizeof(struct clone_args)))
> > +	if (unlikely(usize < CLONE_ARGS_SIZE_VER0))
> >  		return -EINVAL;
> >  
> > -	if (unlikely(!access_ok(uargs, size)))
> > -		return -EFAULT;
> > -
> > -	if (size > sizeof(struct clone_args)) {
> > -		unsigned char __user *addr;
> > -		unsigned char __user *end;
> > -		unsigned char val;
> > -
> > -		addr = (void __user *)uargs + sizeof(struct clone_args);
> > -		end = (void __user *)uargs + size;
> > -
> > -		for (; addr < end; addr++) {
> > -			if (get_user(val, addr))
> > -				return -EFAULT;
> > -			if (val)
> > -				return -E2BIG;
> > -		}
> > -
> > -		size = sizeof(struct clone_args);
> > -	}
> > -
> > -	if (copy_from_user(&args, uargs, size))
> > -		return -EFAULT;
> > +	err = copy_struct_from_user(&args, sizeof(args), uargs, usize);
> > +	if (err)
> > +		return err;
> >  
> >  	/*
> >  	 * Verify that higher 32bits of exit_signal are unset and that
> > -- 
> > 2.23.0
> > 

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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