[PATCH] ima: Check for ERR_PTR from dentry_path() in validate_hash_algo()

[Bradley Morgan <brads@mainlining.org>]

dentry_path() returns ERR_PTR(-ENAMETOOLONG) when the path exceeds the
buffer. validate_hash_algo() passes the result straight to
integrity_audit_msg() without checking. ERR_PTR is not NULL, so
integrity_audit_message() sees a valid pointer and calls strlen() on
it, which faults:

    BUG: unable to handle page fault for address: ffffffffffffffdc
    RIP: 0010:strlen+0x30/0xa0
    Call Trace:
     audit_log_untrustedstring+0x19/0x30
     integrity_audit_message+0x366/0x4f0
     ima_inode_setxattr+0x512/0x5f0

Check for IS_ERR() and use NULL instead, which makes the audit message
skip the name= field instead of crashing.

Fixes: 4f2946aa0c45 ("IMA: introduce a new policy option func=SETXATTR_CHECK")
Cc: stable@vger.kernel.org
Reported-by: syzbot+5ebeb3089ea6439c37be@syzkaller.appspotmail.com
Link: https://lore.kernel.org/all/6a8f89e5.1d9ded08.62e62.00bf.GAE@google.com/
Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 security/integrity/ima/ima_appraise.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index ced2e131b061..b280488e15fc 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -748,6 +748,8 @@ static int validate_hash_algo(struct dentry *dentry,
 		return -EACCES;
 
 	path = dentry_path(dentry, pathbuf, PATH_MAX);
+	if (IS_ERR(path))
+		path = NULL;
 
 	integrity_audit_msg(AUDIT_INTEGRITY_DATA, d_inode(dentry), path,
 			    "set_data", errmsg, -EACCES, 0);
-- 
2.47.3