keys: make keyring key-chunk byte order agree with keyring_diff_objects()

[ Upstream commit 58565eef0f ]

keyring_get_key_chunk() loads description bytes into the index chunk low
address first, while keyring_diff_objects() numbers the first differing
bit from the low end and folds the absolute byte index into the level
without removing the inline-prefix offset the level already carries.
The two disagree on byte order and bit position, so the array can be
told two keys first differ at a bit that does not differ in the chunk
the walker uses, letting crafted descriptions collide into one node.

Load the chunk in the order keyring_diff_objects() assumes and drop the
inline-prefix length when folding the byte index into the level.  This
only changes the in-memory ordering used to place keys within a keyring;
add, search and read of non-colliding keys are unaffected.

Fixes: f771fde820 ("keys: Simplify key description management")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20260719161505.2423935-3-michael.bommarito@gmail.com
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

security/keys/keyring.c

@@ -293,9 +293,10 @@ static unsigned long keyring_get_key_chunk(const void *data, int level)
 		desc_len -= offset;
 		if (desc_len > n)
 			desc_len = n;
+		d += desc_len;
 		do {
 			chunk <<= 8;
-			chunk |= *d++;
+			chunk |= *--d;
 		} while (--desc_len > 0);
 		return chunk;
 	}
@@ -376,7 +377,7 @@ same:
 	return -1;
 
 differ_plus_i:
-	level += i;
+	level += i - (int)sizeof(a->desc);
 differ:
 	i = level * 8 + __ffs(seg_a ^ seg_b);
 	return i;