mseal: fix is_madv_discard()

is_madv_discard did its check wrong. MADV_ flags are not bitwise,
they're normal sequential numbers. So, for instance:
	behavior & (/* ... */ | MADV_REMOVE)

tagged both MADV_REMOVE and MADV_RANDOM (bit 0 set) as discard
operations.

As a result the kernel could erroneously block certain madvises (e.g
MADV_RANDOM or MADV_HUGEPAGE) on sealed VMAs due to them sharing bits
with blocked MADV operations (e.g REMOVE or WIPEONFORK).

This is obviously incorrect, so use a switch statement instead.

Link: https://lkml.kernel.org/r/20240807173336.2523757-1-pedro.falcato@gmail.com
Link: https://lkml.kernel.org/r/20240807173336.2523757-2-pedro.falcato@gmail.com
Fixes: 8be7258aad44 ("mseal: add mseal syscall")
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Tested-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Jeff Xu <jeffxu@chromium.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/mseal.c b/mm/mseal.c
index bf783bba8ed0b92ba80fac5b3459d9bfe0ceb4b4..15bba28acc005fdd8725f4dda8937bbf364433c2 100644 (file)
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -40,9 +40,17 @@ static bool can_modify_vma(struct vm_area_struct *vma)
 
 static bool is_madv_discard(int behavior)
 {
-       return  behavior &
-               (MADV_FREE | MADV_DONTNEED | MADV_DONTNEED_LOCKED |
-                MADV_REMOVE | MADV_DONTFORK | MADV_WIPEONFORK);
+       switch (behavior) {
+       case MADV_FREE:
+       case MADV_DONTNEED:
+       case MADV_DONTNEED_LOCKED:
+       case MADV_REMOVE:
+       case MADV_DONTFORK:
+       case MADV_WIPEONFORK:
+               return true;
+       }
+
+       return false;
 }
 
 static bool is_ro_anon(struct vm_area_struct *vma)