fs/select: Fix memory corruption in compat_get_fd_set()

79de3cbe · Helge Deller <[email protected]> · 2017-08-23 22:37:00 +02:00

Commit 464d62421cb8 ("select: switch compat_{get,put}_fd_set() to
compat_{get,put}_bitmap()") changed the calculation on how many bytes
need to be zeroed when userspace handed over a NULL pointer for a fdset
array in the select syscall.

The calculation was changed in compat_get_fd_set() wrongly from
	memset(fdset, 0, ((nr + 1) & ~1)*sizeof(compat_ulong_t));
to
	memset(fdset, 0, ALIGN(nr, BITS_PER_LONG));

The ALIGN(nr, BITS_PER_LONG) calculates the number of _bits_ which need
to be zeroed in the target fdset array (rounded up to the next full bits
for an unsigned long).

But the memset() call expects the number of _bytes_ to be zeroed.

This leads to clearing more memory than wanted (on the stack area or
even at kmalloc()ed memory areas) and to random kernel crashes as we
have seen them on the parisc platform.

The correct change should have been

	memset(fdset, 0, (ALIGN(nr, BITS_PER_LONG) / BITS_PER_LONG) * BYTES_PER_LONG);

which is the same as can be archieved with a call to

	zero_fd_set(nr, fdset).

Fixes: 464d62421cb8 ("select: switch compat_{get,put}_fd_set() to compat_{get,put}_bitmap()"
Acked-by:: Al Viro <[email protected]>
Signed-off-by: Helge Deller <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Commit: 79de3cbe9a974e03a02b71da80da9ee0eb15a2d0
Parents: 702e9762

Changes

1 files changed · +1 −5

File	Status	Add	Del	Content
fs/select.c	modified	+1	−5	View file

fs/select.c

View file

diff --git a/fs/select.c b/fs/select.c
index 9d5f15e..c6362e3 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -1164,11 +1164,7 @@ int compat_get_fd_set(unsigned long nr, compat_ulong_t __user *ufdset,
 	if (ufdset) {
 		return compat_get_bitmap(fdset, ufdset, nr);
 	} else {
-		/* Tricky, must clear full unsigned long in the
-		 * kernel fdset at the end, ALIGN makes sure that
-		 * actually happens.
-		 */
-		memset(fdset, 0, ALIGN(nr, BITS_PER_LONG));
+		zero_fd_set(nr, fdset);
 		return 0;
 	}
 }