do_remount() does:
mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
mnt->mnt.mnt_flags = mnt_flags;
This can easily be compiled as:
mnt->mnt.mnt_flags &= ~MNT_USER_SETTABLE_MASK;
mnt->mnt.mnt_flags |= mnt_flags;
(also 2 memory accesses, less register pressure)
The flags are being concurrently read by e.g. do_mmap_pgoff()
which does:
if (file->f_path.mnt->mnt_flags & MNT_NOEXEC)
As the result we can allow to mmap a MNT_NOEXEC mount
as VM_EXEC.
Use WRITE_ONCE() to set new flags.
The data race was found with KernelThreadSanitizer (KTSAN).
Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
---
For the record KTSAN report on 4.2:
ThreadSanitizer: data-race in do_mmap_pgoff
Read at 0xffff8800bb857e30 of size 4 by thread 1471 on CPU 0:
[<ffffffff8121e770>] do_mmap_pgoff+0x4f0/0x590 mm/mmap.c:1341
[<ffffffff811f8a6a>] vm_mmap_pgoff+0xaa/0xe0 mm/util.c:297
[<ffffffff811f8b0c>] vm_mmap+0x6c/0x90 mm/util.c:315
[<ffffffff812ea95c>] elf_map+0x13c/0x160 fs/binfmt_elf.c:365
[<ffffffff812eb3cd>] load_elf_binary+0x91d/0x22b0 fs/binfmt_elf.c:955 (discriminator 9)
[<ffffffff8126a712>] search_binary_handler+0x142/0x360 fs/exec.c:1422
[< inline >] exec_binprm fs/exec.c:1464
[<ffffffff8126c689>] do_execveat_common.isra.36+0x919/0xb40 fs/exec.c:1584
[< inline >] do_execve fs/exec.c:1628
[< inline >] SYSC_execve fs/exec.c:1709
[<ffffffff8126cdc6>] SyS_execve+0x46/0x60 fs/exec.c:1704
[<ffffffff81ee4145>] return_from_execve+0x0/0x23 arch/x86/entry/entry_64.S:428
Previous write at 0xffff8800bb857e30 of size 4 by thread 1468 on CPU 8:
[< inline >] do_remount fs/namespace.c:2215
[<ffffffff8129a157>] do_mount+0x637/0x1450 fs/namespace.c:2716
[< inline >] SYSC_mount fs/namespace.c:2915
[<ffffffff8129b4e3>] SyS_mount+0xa3/0x100 fs/namespace.c:2893
[<ffffffff81ee3e11>] entry_SYSCALL_64_fastpath+0x31/0x95 arch/x86/entry/entry_64.S:188
Mutexes locked by thread 1471:
Mutex 228939 is locked here:
[<ffffffff81edf7c2>] mutex_lock_interruptible+0x62/0xa0 kernel/locking/mutex.c:805
[<ffffffff8126bd0f>] prepare_bprm_creds+0x4f/0xb0 fs/exec.c:1172
[<ffffffff8126beaf>] do_execveat_common.isra.36+0x13f/0xb40 fs/exec.c:1517
[< inline >] do_execve fs/exec.c:1628
[< inline >] SYSC_execve fs/exec.c:1709
[<ffffffff8126cdc6>] SyS_execve+0x46/0x60 fs/exec.c:1704
[<ffffffff81ee4145>] return_from_execve+0x0/0x23 arch/x86/entry/entry_64.S:428
Mutex 223016 is locked here:
[<ffffffff81ee0d45>] down_write+0x65/0x80 kernel/locking/rwsem.c:62
[<ffffffff811f8a4c>] vm_mmap_pgoff+0x8c/0xe0 mm/util.c:296
[<ffffffff811f8b0c>] vm_mmap+0x6c/0x90 mm/util.c:315
[<ffffffff812ea95c>] elf_map+0x13c/0x160 fs/binfmt_elf.c:365
[<ffffffff812eb3cd>] load_elf_binary+0x91d/0x22b0 fs/binfmt_elf.c:955 (discriminator 9)
[<ffffffff8126a712>] search_binary_handler+0x142/0x360 fs/exec.c:1422
[< inline >] exec_binprm fs/exec.c:1464
[<ffffffff8126c689>] do_execveat_common.isra.36+0x919/0xb40 fs/exec.c:1584
[< inline >] do_execve fs/exec.c:1628
[< inline >] SYSC_execve fs/exec.c:1709
[<ffffffff8126cdc6>] SyS_execve+0x46/0x60 fs/exec.c:1704
[<ffffffff81ee4145>] return_from_execve+0x0/0x23 arch/x86/entry/entry_64.S:428
Mutexes locked by thread 1468:
Mutex 119619 is locked here:
[<ffffffff81ee0d45>] down_write+0x65/0x80 kernel/locking/rwsem.c:62
[< inline >] do_remount fs/namespace.c:2205
[<ffffffff81299ff1>] do_mount+0x4d1/0x1450 fs/namespace.c:2716
[< inline >] SYSC_mount fs/namespace.c:2915
[<ffffffff8129b4e3>] SyS_mount+0xa3/0x100 fs/namespace.c:2893
[<ffffffff81ee3e11>] entry_SYSCALL_64_fastpath+0x31/0x95 arch/x86/entry/entry_64.S:188
Mutex 193 is locked here:
[< inline >] __raw_spin_lock include/linux/spinlock_api_smp.h:158
[<ffffffff81ee37d0>] _raw_spin_lock+0x50/0x70 kernel/locking/spinlock.c:151
[< inline >] spin_lock include/linux/spinlock.h:312
[< inline >] write_seqlock include/linux/seqlock.h:470
[< inline >] lock_mount_hash fs/mount.h:112
[< inline >] do_remount fs/namespace.c:2213
[<ffffffff8129a10e>] do_mount+0x5ee/0x1450 fs/namespace.c:2716
[< inline >] SYSC_mount fs/namespace.c:2915
[<ffffffff8129b4e3>] SyS_mount+0xa3/0x100 fs/namespace.c:2893
[<ffffffff81ee3e11>] entry_SYSCALL_64_fastpath+0x31/0x95 arch/x86/entry/entry_64.S:188
---
fs/namespace.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 0570729..d0040be 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2212,7 +2212,7 @@ static int do_remount(struct path *path, int flags, int mnt_flags,
if (!err) {
lock_mount_hash();
mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
- mnt->mnt.mnt_flags = mnt_flags;
+ WRITE_ONCE(mnt->mnt.mnt_flags, mnt_flags);
touch_mnt_namespace(mnt->mnt_ns);
unlock_mount_hash();
}
--
2.6.0.rc0.131.gf624c3d
--
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