From: Dave Hansen <[email protected]>
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 <[email protected]>
---
b/ipc/shm.c | 18 +++++++++++++-----
1 file changed, 13 insertions(+), 5 deletions(-)
diff -puN ipc/shm.c~mm-shmdt-fix-over-aggressive-unmap ipc/shm.c
--- a/ipc/shm.c~mm-shmdt-fix-over-aggressive-unmap 2014-11-03
14:32:09.479595152 -0800
+++ b/ipc/shm.c 2014-11-03 16:04:28.340225666 -0800
@@ -1229,6 +1229,7 @@ SYSCALL_DEFINE1(shmdt, char __user *, sh
int retval = -EINVAL;
#ifdef CONFIG_MMU
loff_t size = 0;
+ struct file *file;
struct vm_area_struct *next;
#endif
@@ -1245,7 +1246,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, sh
* 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,
@@ -1271,8 +1273,14 @@ SYSCALL_DEFINE1(shmdt, char __user *, sh
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
@@ -1298,8 +1306,8 @@ SYSCALL_DEFINE1(shmdt, char __user *, sh
/* 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;
}
_
--
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