[PATCH] IMA: set a default value for unknown digsig algorithms

[THOBY Simon <Simon.THOBY@viveris.fr>]

When adding new protections against writing invalid data in
the security.ima xattr, I erroneously expected ima_get_hash_algo()
to always return a valid 'enum hash_algo', but it turns out it trusts
the user-supplied digital signatures and return it without any checks.
It didn't affect process_measurement() because that function
(indirectly) calls into ima_alloc_atfm() that fallback silently
on the default hash algorithm, but it did affect ima_inode_setxattr
as that new function didn't perform a bounds check.

Update ima_get_hash_algo() to always return a valid hash algorithm,
defaulting on 'ima_hash_algo' when the user-supplied value inside
the xattr is invalid.

This patch was successfully tested by syszbot, see
https://syzkaller.appspot.com/bug?extid=e8bafe7b82c739eaf153.

Signed-off-by: THOBY Simon <Simon.THOBY@viveris.fr>
Reported-by: syzbot+e8bafe7b82c739eaf153@syzkaller.appspotmail.com
---
 security/integrity/ima/ima_appraise.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 8f1eb7ef041e..dbba51583e7c 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -185,7 +185,8 @@ enum hash_algo ima_get_hash_algo(const struct evm_ima_xattr_data *xattr_value,
 	switch (xattr_value->type) {
 	case EVM_IMA_XATTR_DIGSIG:
 		sig = (typeof(sig))xattr_value;
-		if (sig->version != 2 || xattr_len <= sizeof(*sig))
+		if (sig->version != 2 || xattr_len <= sizeof(*sig)
+		    || sig->hash_algo >= HASH_ALGO__LAST)
 			return ima_hash_algo;
 		return sig->hash_algo;
 		break;
-- 
2.31.1