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Revert "ipc/shm: Fix shmat mmap nil-page protection"

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Patch series "ipc/shm: shmat() fixes around nil-page".

These patches fix two issues reported[1] a while back by Joe and Andrea
around how shmat(2) behaves with nil-page.

The first reverts a commit that it was incorrectly thought that mapping
nil-page (address=0) was a no no with MAP_FIXED.  This is not the case,
with the exception of SHM_REMAP; which is address in the second patch.

I chose two patches because it is easier to backport and it explicitly
reverts bogus behaviour.  Both patches ought to be in -stable and ltp
testcases need updated (the added testcase around the cve can be
modified to just test for SHM_RND|SHM_REMAP).

[1] lkml.kernel.org/r/20180430172152.nfa564pvgpk3ut7p@linux-n805

This patch (of 2):

Commit 95e91b831f ("ipc/shm: Fix shmat mmap nil-page protection")
worked on the idea that we should not be mapping as root addr=0 and
MAP_FIXED.  However, it was reported that this scenario is in fact
valid, thus making the patch both bogus and breaks userspace as well.

For example X11's libint10.so relies on shmat(1, SHM_RND) for lowmem
initialization[1].

[1] https://cgit.freedesktop.org/xorg/xserver/tree/hw/xfree86/os-support/linux/int10/linux.c#n347
Link: http://lkml.kernel.org/r/[email protected]
Fixes: 95e91b831f ("ipc/shm: Fix shmat mmap nil-page protection")
Signed-off-by: Davidlohr Bueso <[email protected]>
Reported-by: Joe Lawrence <[email protected]>
Reported-by: Andrea Arcangeli <[email protected]>
Cc: Manfred Spraul <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Davidlohr Bueso 2018-05-25 14:47:27 -07:00 • committed by Linus Torvalds
commit a73ab244f0
1 changed files with 2 additions and 7 deletions
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9
ipc/shm.c
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@ -1363,13 +1363,8 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
if (addr) {
if (addr & (shmlba - 1)) {
/*
* Round down to the nearest multiple of shmlba.
* For sane do_mmap_pgoff() parameters, avoid
* round downs that trigger nil-page and MAP_FIXED.
*/
if ((shmflg & SHM_RND) && addr >= shmlba)
addr &= ~(shmlba - 1);
if (shmflg & SHM_RND)
addr &= ~(shmlba - 1); /* round down */
else
#ifndef __ARCH_FORCE_SHMLBA
if (addr & ~PAGE_MASK)