[PATCH 4/4] mseal: fix is_madv_discard()

[jeffxu at chromium.org]

From: Pedro Falcato <pedro.falcato at gmail.com>

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
Signed-off-by: Pedro Falcato <pedro.falcato at gmail.com>
Tested-by: Jeff Xu <jeffxu at chromium.org>
Reviewed-by: Jeff Xu <jeffxu at chromium.org>
Cc: Kees Cook <kees at kernel.org>
Cc: Liam R. Howlett <Liam.Howlett at oracle.com>
Cc: Shuah Khan <shuah at kernel.org>
Cc: <stable at vger.kernel.org>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
(cherry picked from commit e46bc2e7eb90a370bc27fa2fd98cb8251e7da1ec)

Signed-off-by: Jeff Xu <jeffxu at chromium.org>

---
 mm/mseal.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/mm/mseal.c b/mm/mseal.c
index bf783bba8ed0..15bba28acc00 100644
--- 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)
-- 
2.47.0.338.g60cca15819-goog