[PATCH 5.10.y] ima: fix out-of-bounds read in xattr_verify()

[Lincoln Wallace <locnnil0@gmail.com>]

The digest-length check in xattr_verify() mixes int and size_t:

	if (xattr_len - sizeof(xattr_value->type) - hash_start >=
			iint->ima_hash->length)

sizeof() yields size_t, so the usual arithmetic conversions promote
the whole left-hand side to unsigned 64-bit before the subtraction
runs. For a truncated xattr this underflows instead of going negative:
a 1-byte IMA_XATTR_DIGEST_NG xattr (xattr_len == 1, hash_start == 1)
turns "1 - 1 - 1" into SIZE_MAX, which is trivially >= ima_hash->length.
The check then passes and the following memcmp() reads
iint->ima_hash->length bytes starting past the end of the buffer
vfs_getxattr_alloc() allocated for it.

Nothing upstream clamps xattr_len back into a safe range first:
ima_get_hash_algo() only special-cases xattr_len < 2 to pick a default
algorithm, and evm_verifyxattr() returns INTEGRITY_UNKNOWN rather than
failing when no HMAC key is loaded, so a truncated security.ima value
reaches the length check as-is.

Rewrite the comparison so every operand stays a signed int and no
implicit conversion to size_t can occur.

Fixes: 3ea7a56067e6 ("ima: provide hash algo info in the xattr")
Cc: stable@vger.kernel.org
Signed-off-by: Lincoln Wallace <locnnil0@gmail.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
(cherry picked from commit 5ff232d31106f45ac87c3b64e1d35a0667777797)
Signed-off-by: Lincoln Wallace <locnnil0@gmail.com>
---
Context conflict only: the IMA_XATTR_DIGEST case was restructured
after 5.10 by commit 7aa5783d9564 ("ima: Allow imasig requirement to be
satisfied by EVM portable signatures"). The fix itself is identical to
upstream.

 security/integrity/ima/ima_appraise.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 7122a359a268..615a20b9eb52 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -242,8 +242,13 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 			break;
 		}
 		clear_bit(IMA_DIGSIG, &iint->atomic_flags);
-		if (xattr_len - sizeof(xattr_value->type) - hash_start >=
-				iint->ima_hash->length)
+		/*
+		 * Use addition, not subtraction: sizeof() forces unsigned
+		 * math and a short xattr_len would wrap around, bypassing
+		 * this bounds check.
+		 */
+		if (xattr_len >= (int)sizeof(xattr_value->type) + hash_start +
+				(int)iint->ima_hash->length)
 			/*
 			 * xattr length may be longer. md5 hash in previous
 			 * version occupied 20 bytes in xattr, instead of 16
-- 
2.53.0