[PATCH 0/4] lib/iov_iter: Fixes and documentation for pmem user copies

[PATCH 0/4] lib/iov_iter: Fixes and documentation for pmem user copies

[Dan Williams]

* Add missing documentation for the 'flushcache' operation for writing to
  persistent memory, and the 'mcsafe' operation for reading from
  potentially poisoned persistent memory.

* Introduce copy_pipe_to_iter_mcsafe() to properly handle ITER_PIPE for
  _copy_to_iter_mcsafe().

* Make sure copy_to_user_mcsafe() falls back to an exception handling
  capable copy routine, not plain memcpy().

---

Dan Williams (4):
      lib/iov_iter: Document _copy_to_iter_mcsafe()
      lib/iov_iter: Document _copy_to_iter_flushcache()
      lib/iov_iter: Fix pipe handling in _copy_to_iter_mcsafe
      x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling


 arch/x86/Kconfig                  |    2 -
 arch/x86/include/asm/uaccess_64.h |    7 +++
 lib/iov_iter.c                    |   77 +++++++++++++++++++++++++++++++++++--
 3 files changed, 80 insertions(+), 6 deletions(-)

[PATCH 1/4] lib/iov_iter: Document _copy_to_iter_mcsafe()

[Dan Williams]

Add some theory of operation documentation to _copy_to_iter_mcsafe().

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 lib/iov_iter.c |   26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 7e43cd54c84c..94fa361be7bb 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -596,6 +596,32 @@ static unsigned long memcpy_mcsafe_to_page(struct page *page, size_t offset,
 	return ret;
 }
 
+/**
+ * _copy_to_iter_mcsafe - copy to user with source-read error exception handling
+ * @addr: source kernel address
+ * @bytes: total transfer length
+ * @iter: destination iterator
+ *
+ * The pmem driver arranges for filesystem-dax to use this facility via
+ * dax_copy_to_iter() for protecting read/write to persistent memory.
+ * Unless / until an architecture can guarantee identical performance
+ * between _copy_to_iter_mcsafe() and _copy_to_iter() it would be a
+ * performance regression to switch more users to the mcsafe version.
+ *
+ * Otherwise, the main differences between this and typical _copy_to_iter().
+ *
+ * * Typical tail/residue handling after a fault retries the copy
+ *   byte-by-byte until the fault happens again. Re-triggering machine
+ *   checks is potentially fatal so the implementation uses source
+ *   alignment and poison alignment assumptions to avoid re-triggering
+ *   hardware exceptions.
+ *
+ * * ITER_KVEC, ITER_PIPE, and ITER_BVEC can return short copies.
+ *   Compare to copy_to_iter() where only ITER_IOVEC attempts might return
+ *   a short copy.
+ *
+ * See MCSAFE_TEST for self-test.
+ */
 size_t _copy_to_iter_mcsafe(const void *addr, size_t bytes, struct iov_iter *i)
 {
 	const char *from = addr;

[PATCH 2/4] lib/iov_iter: Document _copy_to_iter_flushcache()

[Dan Williams]

Add some theory of operation documentation to _copy_to_iter_flushcache().

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 lib/iov_iter.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 94fa361be7bb..09fb73ad9d54 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -727,6 +727,20 @@ size_t _copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 EXPORT_SYMBOL(_copy_from_iter_nocache);
 
 #ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
+/**
+ * _copy_from_iter_flushcache - write destination through cpu cache
+ * @addr: destination kernel address
+ * @bytes: total transfer length
+ * @iter: source iterator
+ *
+ * The pmem driver arranges for filesystem-dax to use this facility via
+ * dax_copy_from_iter() for ensuring that writes to persistent memory
+ * are flushed through the CPU cache. It is differentiated from
+ * _copy_from_iter_nocache() in that guarantees all data is flushed for
+ * all iterator types. The _copy_from_iter_nocache() only attempts to
+ * bypass the cache for the ITER_IOVEC case, and on some archs may use
+ * instructions that strand dirty-data in the cache.
+ */
 size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)
 {
 	char *to = addr;

[PATCH 3/4] lib/iov_iter: Fix pipe handling in _copy_to_iter_mcsafe

[Dan Williams]

By mistake the ITER_PIPE early-exit / warning from copy_from_iter() was
cargo-culted in _copy_to_iter_mcsafe() rather than a machine-check-safe
version of copy_to_iter_pipe().

Implement copy_pipe_to_iter_mcsafe() being careful to return the
indication of short copies due to a CPU exception.

Without this regression-fix all splice reads to dax-mode files fail.

Fixes: 8780356ef630 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 lib/iov_iter.c |   37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 09fb73ad9d54..8be175df3075 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -596,6 +596,37 @@ static unsigned long memcpy_mcsafe_to_page(struct page *page, size_t offset,
 	return ret;
 }
 
+static size_t copy_pipe_to_iter_mcsafe(const void *addr, size_t bytes,
+				struct iov_iter *i)
+{
+	struct pipe_inode_info *pipe = i->pipe;
+	size_t n, off, xfer = 0;
+	int idx;
+
+	if (!sanity(i))
+		return 0;
+
+	bytes = n = push_pipe(i, bytes, &idx, &off);
+	if (unlikely(!n))
+		return 0;
+	for ( ; n; idx = next_idx(idx, pipe), off = 0) {
+		size_t chunk = min_t(size_t, n, PAGE_SIZE - off);
+		unsigned long rem;
+
+		rem = memcpy_mcsafe_to_page(pipe->bufs[idx].page, off, addr,
+				chunk);
+		i->idx = idx;
+		i->iov_offset = off + chunk - rem;
+		xfer += chunk - rem;
+		if (rem)
+			break;
+		n -= chunk;
+		addr += chunk;
+	}
+	i->count -= xfer;
+	return xfer;
+}
+
 /**
  * _copy_to_iter_mcsafe - copy to user with source-read error exception handling
  * @addr: source kernel address
@@ -627,10 +658,8 @@ size_t _copy_to_iter_mcsafe(const void *addr, size_t bytes, struct iov_iter *i)
 	const char *from = addr;
 	unsigned long rem, curr_addr, s_addr = (unsigned long) addr;
 
-	if (unlikely(i->type & ITER_PIPE)) {
-		WARN_ON(1);
-		return 0;
-	}
+	if (unlikely(i->type & ITER_PIPE))
+		return copy_pipe_to_iter_mcsafe(addr, bytes, i);
 	if (iter_is_iovec(i))
 		might_fault();
 	iterate_and_advance(i, bytes, v,

[PATCH 4/4] x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling

[Dan Williams]

All copy_to_user() implementations need to be prepared to handle faults
accessing userspace. The __memcpy_mcsafe() implementation handles both
mmu-faults on the user destination and machine-check-exceptions on the
source buffer. However, the memcpy_mcsafe() wrapper may silently
fallback to memcpy() depending on build options and cpu-capabilities.

Force copy_to_user_mcsafe() to always use __memcpy_mcsafe() when
available, and otherwise disable all of the copy_to_user_mcsafe()
infrastructure when __memcpy_mcsafe() is not available, i.e.
CONFIG_X86_MCE=n.

This fixes crashes of the form:
    run fstests generic/323 at 2018-07-02 12:46:23
    BUG: unable to handle kernel paging request at 00007f0d50001000
    RIP: 0010:__memcpy+0x12/0x20
    [..]
    Call Trace:
     copyout_mcsafe+0x3a/0x50
     _copy_to_iter_mcsafe+0xa1/0x4a0
     ? dax_alive+0x30/0x50
     dax_iomap_actor+0x1f9/0x280
     ? dax_iomap_rw+0x100/0x100
     iomap_apply+0xba/0x130
     ? dax_iomap_rw+0x100/0x100
     dax_iomap_rw+0x95/0x100
     ? dax_iomap_rw+0x100/0x100
     xfs_file_dax_read+0x7b/0x1d0 [xfs]
     xfs_file_read_iter+0xa7/0xc0 [xfs]
     aio_read+0x11c/0x1a0

Fixes: 8780356ef630 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 arch/x86/Kconfig                  |    2 +-
 arch/x86/include/asm/uaccess_64.h |    7 ++++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f1dbb4ee19d7..887d3a7bb646 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -63,7 +63,7 @@ config X86
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_REFCOUNT
 	select ARCH_HAS_UACCESS_FLUSHCACHE	if X86_64
-	select ARCH_HAS_UACCESS_MCSAFE		if X86_64
+	select ARCH_HAS_UACCESS_MCSAFE		if X86_64 && X86_MCE
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAS_STRICT_KERNEL_RWX
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 62acb613114b..a9d637bc301d 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -52,7 +52,12 @@ copy_to_user_mcsafe(void *to, const void *from, unsigned len)
 	unsigned long ret;
 
 	__uaccess_begin();
-	ret = memcpy_mcsafe(to, from, len);
+	/*
+	 * Note, __memcpy_mcsafe() is explicitly used since it can
+	 * handle exceptions / faults.  memcpy_mcsafe() may fall back to
+	 * memcpy() which lacks this handling.
+	 */
+	ret = __memcpy_mcsafe(to, from, len);
 	__uaccess_end();
 	return ret;
 }

[tip:core/urgent] lib/iov_iter: Document _copy_to_iter_mcsafe()

[tip-bot for Dan Williams]

Commit-ID:  bf3eeb9b5f2a1a05b3a68c6d82112babd58d6a39
Gitweb:     https://git.kernel.org/tip/bf3eeb9b5f2a1a05b3a68c6d82112babd58d6a39
Author:     Dan Williams <dan.j.williams@intel.com>
AuthorDate: Sun, 8 Jul 2018 13:46:02 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 16 Jul 2018 00:05:05 +0200

lib/iov_iter: Document _copy_to_iter_mcsafe()

Add some theory of operation documentation to _copy_to_iter_mcsafe().

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/153108276256.37979.1689794213845539316.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 lib/iov_iter.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 7e43cd54c84c..94fa361be7bb 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -596,6 +596,32 @@ static unsigned long memcpy_mcsafe_to_page(struct page *page, size_t offset,
 	return ret;
 }
 
+/**
+ * _copy_to_iter_mcsafe - copy to user with source-read error exception handling
+ * @addr: source kernel address
+ * @bytes: total transfer length
+ * @iter: destination iterator
+ *
+ * The pmem driver arranges for filesystem-dax to use this facility via
+ * dax_copy_to_iter() for protecting read/write to persistent memory.
+ * Unless / until an architecture can guarantee identical performance
+ * between _copy_to_iter_mcsafe() and _copy_to_iter() it would be a
+ * performance regression to switch more users to the mcsafe version.
+ *
+ * Otherwise, the main differences between this and typical _copy_to_iter().
+ *
+ * * Typical tail/residue handling after a fault retries the copy
+ *   byte-by-byte until the fault happens again. Re-triggering machine
+ *   checks is potentially fatal so the implementation uses source
+ *   alignment and poison alignment assumptions to avoid re-triggering
+ *   hardware exceptions.
+ *
+ * * ITER_KVEC, ITER_PIPE, and ITER_BVEC can return short copies.
+ *   Compare to copy_to_iter() where only ITER_IOVEC attempts might return
+ *   a short copy.
+ *
+ * See MCSAFE_TEST for self-test.
+ */
 size_t _copy_to_iter_mcsafe(const void *addr, size_t bytes, struct iov_iter *i)
 {
 	const char *from = addr;

[tip:core/urgent] lib/iov_iter: Document _copy_to_iter_flushcache()

[tip-bot for Dan Williams]

Commit-ID:  abd08d7d245397bcbded8c6c29ff79a36b3875b0
Gitweb:     https://git.kernel.org/tip/abd08d7d245397bcbded8c6c29ff79a36b3875b0
Author:     Dan Williams <dan.j.williams@intel.com>
AuthorDate: Sun, 8 Jul 2018 13:46:07 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 16 Jul 2018 00:05:05 +0200

lib/iov_iter: Document _copy_to_iter_flushcache()

Add some theory of operation documentation to _copy_to_iter_flushcache().

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/153108276767.37979.9462477994086841699.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 lib/iov_iter.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 94fa361be7bb..09fb73ad9d54 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -727,6 +727,20 @@ size_t _copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
 EXPORT_SYMBOL(_copy_from_iter_nocache);
 
 #ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
+/**
+ * _copy_from_iter_flushcache - write destination through cpu cache
+ * @addr: destination kernel address
+ * @bytes: total transfer length
+ * @iter: source iterator
+ *
+ * The pmem driver arranges for filesystem-dax to use this facility via
+ * dax_copy_from_iter() for ensuring that writes to persistent memory
+ * are flushed through the CPU cache. It is differentiated from
+ * _copy_from_iter_nocache() in that guarantees all data is flushed for
+ * all iterator types. The _copy_from_iter_nocache() only attempts to
+ * bypass the cache for the ITER_IOVEC case, and on some archs may use
+ * instructions that strand dirty-data in the cache.
+ */
 size_t _copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)
 {
 	char *to = addr;

[tip:core/urgent] lib/iov_iter: Fix pipe handling in _copy_to_iter_mcsafe()

[tip-bot for Dan Williams]

Commit-ID:  ca146f6f091e47b3fd18d6a7e76ec0297d202e0f
Gitweb:     https://git.kernel.org/tip/ca146f6f091e47b3fd18d6a7e76ec0297d202e0f
Author:     Dan Williams <dan.j.williams@intel.com>
AuthorDate: Sun, 8 Jul 2018 13:46:12 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 16 Jul 2018 00:05:05 +0200

lib/iov_iter: Fix pipe handling in _copy_to_iter_mcsafe()

By mistake the ITER_PIPE early-exit / warning from copy_from_iter() was
cargo-culted in _copy_to_iter_mcsafe() rather than a machine-check-safe
version of copy_to_iter_pipe().

Implement copy_pipe_to_iter_mcsafe() being careful to return the
indication of short copies due to a CPU exception.

Without this regression-fix all splice reads to dax-mode files fail.

Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Fixes: 8780356ef630 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Link: http://lkml.kernel.org/r/153108277278.37979.3327916996902264102.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 lib/iov_iter.c | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 09fb73ad9d54..8be175df3075 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -596,6 +596,37 @@ static unsigned long memcpy_mcsafe_to_page(struct page *page, size_t offset,
 	return ret;
 }
 
+static size_t copy_pipe_to_iter_mcsafe(const void *addr, size_t bytes,
+				struct iov_iter *i)
+{
+	struct pipe_inode_info *pipe = i->pipe;
+	size_t n, off, xfer = 0;
+	int idx;
+
+	if (!sanity(i))
+		return 0;
+
+	bytes = n = push_pipe(i, bytes, &idx, &off);
+	if (unlikely(!n))
+		return 0;
+	for ( ; n; idx = next_idx(idx, pipe), off = 0) {
+		size_t chunk = min_t(size_t, n, PAGE_SIZE - off);
+		unsigned long rem;
+
+		rem = memcpy_mcsafe_to_page(pipe->bufs[idx].page, off, addr,
+				chunk);
+		i->idx = idx;
+		i->iov_offset = off + chunk - rem;
+		xfer += chunk - rem;
+		if (rem)
+			break;
+		n -= chunk;
+		addr += chunk;
+	}
+	i->count -= xfer;
+	return xfer;
+}
+
 /**
  * _copy_to_iter_mcsafe - copy to user with source-read error exception handling
  * @addr: source kernel address
@@ -627,10 +658,8 @@ size_t _copy_to_iter_mcsafe(const void *addr, size_t bytes, struct iov_iter *i)
 	const char *from = addr;
 	unsigned long rem, curr_addr, s_addr = (unsigned long) addr;
 
-	if (unlikely(i->type & ITER_PIPE)) {
-		WARN_ON(1);
-		return 0;
-	}
+	if (unlikely(i->type & ITER_PIPE))
+		return copy_pipe_to_iter_mcsafe(addr, bytes, i);
 	if (iter_is_iovec(i))
 		might_fault();
 	iterate_and_advance(i, bytes, v,

[tip:core/urgent] x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling

[tip-bot for Dan Williams]

Commit-ID:  092b31aa2048cf7561a39697974adcd147fbb27b
Gitweb:     https://git.kernel.org/tip/092b31aa2048cf7561a39697974adcd147fbb27b
Author:     Dan Williams <dan.j.williams@intel.com>
AuthorDate: Sun, 8 Jul 2018 13:46:17 -0700
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 16 Jul 2018 00:05:05 +0200

x86/asm/memcpy_mcsafe: Fix copy_to_user_mcsafe() exception handling

All copy_to_user() implementations need to be prepared to handle faults
accessing userspace. The __memcpy_mcsafe() implementation handles both
mmu-faults on the user destination and machine-check-exceptions on the
source buffer. However, the memcpy_mcsafe() wrapper may silently
fallback to memcpy() depending on build options and cpu-capabilities.

Force copy_to_user_mcsafe() to always use __memcpy_mcsafe() when
available, and otherwise disable all of the copy_to_user_mcsafe()
infrastructure when __memcpy_mcsafe() is not available, i.e.
CONFIG_X86_MCE=n.

This fixes crashes of the form:
    run fstests generic/323 at 2018-07-02 12:46:23
    BUG: unable to handle kernel paging request at 00007f0d50001000
    RIP: 0010:__memcpy+0x12/0x20
    [..]
    Call Trace:
     copyout_mcsafe+0x3a/0x50
     _copy_to_iter_mcsafe+0xa1/0x4a0
     ? dax_alive+0x30/0x50
     dax_iomap_actor+0x1f9/0x280
     ? dax_iomap_rw+0x100/0x100
     iomap_apply+0xba/0x130
     ? dax_iomap_rw+0x100/0x100
     dax_iomap_rw+0x95/0x100
     ? dax_iomap_rw+0x100/0x100
     xfs_file_dax_read+0x7b/0x1d0 [xfs]
     xfs_file_read_iter+0xa7/0xc0 [xfs]
     aio_read+0x11c/0x1a0

Reported-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Tested-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Fixes: 8780356ef630 ("x86/asm/memcpy_mcsafe: Define copy_to_iter_mcsafe()")
Link: http://lkml.kernel.org/r/153108277790.37979.1486841789275803399.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/Kconfig                  | 2 +-
 arch/x86/include/asm/uaccess_64.h | 7 ++++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index f1dbb4ee19d7..887d3a7bb646 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -63,7 +63,7 @@ config X86
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_REFCOUNT
 	select ARCH_HAS_UACCESS_FLUSHCACHE	if X86_64
-	select ARCH_HAS_UACCESS_MCSAFE		if X86_64
+	select ARCH_HAS_UACCESS_MCSAFE		if X86_64 && X86_MCE
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAS_STRICT_KERNEL_RWX
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index 62acb613114b..a9d637bc301d 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -52,7 +52,12 @@ copy_to_user_mcsafe(void *to, const void *from, unsigned len)
 	unsigned long ret;
 
 	__uaccess_begin();
-	ret = memcpy_mcsafe(to, from, len);
+	/*
+	 * Note, __memcpy_mcsafe() is explicitly used since it can
+	 * handle exceptions / faults.  memcpy_mcsafe() may fall back to
+	 * memcpy() which lacks this handling.
+	 */
+	ret = __memcpy_mcsafe(to, from, len);
 	__uaccess_end();
 	return ret;
 }