linux-kernel - [PATCH 3.12 077/155] ipc/shm.c: fix overly aggressive shmdt() when calls span multiple segments

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Message-Id: <2e229b9bb75f98ec2fe7d5b0ff998d293c5ad410.1428411004.git.jslaby@suse.cz>
Date:	Tue,  7 Apr 2015 14:50:46 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 077/155] ipc/shm.c: fix overly aggressive shmdt() when calls span multiple segments

From: Dave Hansen <dave.hansen@...ux.intel.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit d3c97900b427b8d5a476fdfe484267f09df418d6 upstream.

This is a highly-contrived scenario.  But, a single shmdt() call can be
induced in to unmapping memory from mulitple shm segments.  Example code
is here:

	http://www.sr71.net/~dave/intel/shmfun.c

The fix is pretty simple: Record the 'struct file' for the first VMA we
encounter and then stick to it.  Decline to unmap anything not from the
same file and thus the same segment.

I found this by inspection and the odds of anyone hitting this in practice
are pretty darn small.

Lightly tested, but it's a pretty small patch.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Manfred Spraul <manfred@...orfullife.com>
Reviewed-by: Davidlohr Bueso <dave@...olabs.net>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 ipc/shm.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 7a51443a51d6..b039a85e2b8d 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1227,6 +1227,7 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 	int retval = -EINVAL;
 #ifdef CONFIG_MMU
 	loff_t size = 0;
+	struct file *file;
 	struct vm_area_struct *next;
 #endif
 
@@ -1243,7 +1244,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 	 *   started at address shmaddr. It records it's size and then unmaps
 	 *   it.
 	 * - Then it unmaps all shm vmas that started at shmaddr and that
-	 *   are within the initially determined size.
+	 *   are within the initially determined size and that are from the
+	 *   same shm segment from which we determined the size.
 	 * Errors from do_munmap are ignored: the function only fails if
 	 * it's called with invalid parameters or if it's called to unmap
 	 * a part of a vma. Both calls in this function are for full vmas,
@@ -1269,8 +1271,14 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 		if ((vma->vm_ops == &shm_vm_ops) &&
 			(vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) {
 
-
-			size = file_inode(vma->vm_file)->i_size;
+			/*
+			 * Record the file of the shm segment being
+			 * unmapped.  With mremap(), someone could place
+			 * page from another segment but with equal offsets
+			 * in the range we are unmapping.
+			 */
+			file = vma->vm_file;
+			size = file_inode(file)->i_size;
 			do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
 			/*
 			 * We discovered the size of the shm segment, so
@@ -1296,8 +1304,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 
 		/* finding a matching vma now does not alter retval */
 		if ((vma->vm_ops == &shm_vm_ops) &&
-			(vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff)
-
+		    ((vma->vm_start - addr)/PAGE_SIZE == vma->vm_pgoff) &&
+		    (vma->vm_file == file))
 			do_munmap(mm, vma->vm_start, vma->vm_end - vma->vm_start);
 		vma = next;
 	}
-- 
2.3.4

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