[PATCH v10 2/2] mm/mremap: optionally randomize mremap(..., MREMAP_MAYMOVE)

[Topi Miettinen <toiwoton@gmail.com>]

New sysctl kernel.randomize_mremap, when set, can be used to force
mremap(..., MREMAP_MAYMOVE) to always move the mappings even if not
necessary. In addition to improved address space layout randomization,
this can expose bugs where the caller is not actually expecting a
moved mapping, even though this may sometimes happen without this
flag.

Example:
$ cat mremap.c
 #define _GNU_SOURCE
 #include <stddef.h>
 #include <sys/mman.h>

int main(void) {
        void *addr = mmap(NULL, 4096, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        addr = mremap(addr, 4096, 8192, MREMAP_MAYMOVE);
        mremap(addr, 4096, 4096, MREMAP_MAYMOVE);
        return 0;
}
$ gcc -o mremap mremap.c
$ strace -e mmap,mremap ./mremap
mmap(NULL, 4096, PROT_READ, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x68a16298d000
mremap(0x68a16298d000, 4096, 8192, MREMAP_MAYMOVE) = 0x68a16298d000
mremap(0x68a16298d000, 4096, 4096, MREMAP_MAYMOVE) = 0x68a16298d000

Setting the sysctl enables randomization:
$ sudo sysctl kernel.randomize_mremap=1
$ strace -e mmap,mremap ./mremap
mmap(NULL, 4096, PROT_READ, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x6366429cf000
mremap(0x6366429cf000, 4096, 8192, MREMAP_MAYMOVE) = 0x70aa47ad0000
mremap(0x70aa47ad0000, 4096, 4096, MREMAP_MAYMOVE) = 0x5b37dc166000

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Jann Horn <jannh@google.com>
CC: Kees Cook <keescook@chromium.org>
CC: Matthew Wilcox <willy@infradead.org>
CC: Mike Rapoport <rppt@kernel.org>
CC: Linux API <linux-api@vger.kernel.org>
Signed-off-by: Topi Miettinen <toiwoton@gmail.com>
---
 Documentation/admin-guide/sysctl/kernel.rst |  9 +++++++
 include/linux/mm.h                          |  2 ++
 kernel/sysctl.c                             |  7 ++++++
 mm/mremap.c                                 | 26 +++++++++++++++++++--
 4 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index c13f865c806c..eeca8c8f96d0 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1029,6 +1029,15 @@ defined, these additional entries are present:
   number of cycles between interrupts used to feed the pool.
 
 
+randomize_mremap
+==================
+
+This option, when set, can be used to force mremap(...,
+MREMAP_MAYMOVE) to always move the mappings even if not necessary.
+In addition to improved address space layout randomization, this can
+expose bugs where the caller is not actually expecting a moved
+mapping, even though this may sometimes happen without this flag.
+
 randomize_va_space
 ==================
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index b4915412abbe..98aa466c2901 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2990,6 +2990,8 @@ void drop_slab_node(int nid);
 extern int randomize_va_space;
 #endif
 
+extern int randomize_mremap;
+
 const char * arch_vma_name(struct vm_area_struct *vma);
 #ifdef CONFIG_MMU
 void print_vma_addr(char *prefix, unsigned long rip);
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index afad085960b8..02bd9ba89f27 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2429,6 +2429,13 @@ static struct ctl_table kern_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
+	{
+		.procname	= "randomize_mremap",
+		.data		= &randomize_mremap,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
 #endif
 #if defined(CONFIG_S390) && defined(CONFIG_SMP)
 	{
diff --git a/mm/mremap.c b/mm/mremap.c
index 138abbae4f75..386da905f39f 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -648,6 +648,14 @@ static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
 	return 1;
 }
 
+/*
+ * Force mremap(..., MREMAP_MAYMOVE) to always move the mappings even
+ * if not necessary. This can expose bugs where the caller is not
+ * actually expecting a moved mapping, even though this may sometimes
+ * happen without this flag.
+ */
+int randomize_mremap __read_mostly = 0;
+
 /*
  * Expand (or shrink) an existing mapping, potentially moving it at the
  * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
@@ -665,6 +673,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	unsigned long charged = 0;
 	bool locked = false;
 	bool downgraded = false;
+	bool randomize = false;
 	struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
 	LIST_HEAD(uf_unmap_early);
 	LIST_HEAD(uf_unmap);
@@ -720,6 +729,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 		goto out;
 	}
 
+	randomize = (flags & MREMAP_MAYMOVE) && randomize_mremap;
 	/*
 	 * Always allow a shrinking remap: that just unmaps
 	 * the unnecessary pages..
@@ -730,7 +740,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 		int retval;
 
 		retval = __do_munmap(mm, addr+new_len, old_len - new_len,
-				  &uf_unmap, true);
+				     &uf_unmap, !randomize);
 		if (retval < 0 && old_len != new_len) {
 			ret = retval;
 			goto out;
@@ -738,6 +748,16 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 		} else if (retval == 1)
 			downgraded = true;
 		ret = addr;
+
+		/*
+		 * Caller is happy with a new address, so let's move
+		 * even if not necessary
+		 */
+		if (randomize)
+			ret = mremap_to(addr, new_len, 0, new_len,
+					&locked, flags, &uf, &uf_unmap_early,
+					&uf_unmap);
+
 		goto out;
 	}
 
@@ -751,8 +771,10 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	}
 
 	/* old_len exactly to the end of the area..
+	 * But when randomizing, don't just expand the mapping if
+	 * caller is happy with a moved and resized mapping
 	 */
-	if (old_len == vma->vm_end - addr) {
+	if (old_len == vma->vm_end - addr && !randomize) {
 		/* can we just expand the current mapping? */
 		if (vma_expandable(vma, new_len - old_len)) {
 			int pages = (new_len - old_len) >> PAGE_SHIFT;
-- 
2.29.2