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apparmor: fix differential encoding verification

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Differential encoding allows loops to be created if it is abused. To
prevent this the unpack should verify that a diff-encode chain
terminates.

Unfortunately the differential encode verification had two bugs.

1. it conflated states that had gone through check and already been
   marked, with states that were currently being checked and marked.
   This means that loops in the current chain being verified are treated
   as a chain that has already been verified.

2. the order bailout on already checked states compared current chain
   check iterators j,k instead of using the outer loop iterator i.
   Meaning a step backwards in states in the current chain verification
   was being mistaken for moving to an already verified state.

Move to a double mark scheme where already verified states get a
different mark, than the current chain being kept. This enables us
to also drop the backwards verification check that was the cause of
the second error as any already verified state is already marked.

Fixes: 031dcc8f4e ("apparmor: dfa add support for state differential encoding")
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Tested-by: Salvatore Bonaccorso <carnil@debian.org>
Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Reviewed-by: Cengiz Can <cengiz.can@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
This commit is contained in:
John Johansen
2025-10-17 01:53:00 -07:00
parent 6601e13e82
commit 39440b1375
2 changed files with 20 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

1
security/apparmor/include/match.h
View File

@@ -185,6 +185,7 @@ static inline void aa_put_dfa(struct aa_dfa *dfa)
#define MATCH_FLAG_DIFF_ENCODE 0x80000000
#define MARK_DIFF_ENCODE 0x40000000
#define MATCH_FLAG_OOB_TRANSITION 0x20000000
#define MARK_DIFF_ENCODE_VERIFIED 0x10000000
#define MATCH_FLAGS_MASK 0xff000000
#define MATCH_FLAGS_VALID (MATCH_FLAG_DIFF_ENCODE | MATCH_FLAG_OOB_TRANSITION)
#define MATCH_FLAGS_INVALID (MATCH_FLAGS_MASK & ~MATCH_FLAGS_VALID)

23
security/apparmor/match.c
View File

@@ -202,16 +202,31 @@ static int verify_dfa(struct aa_dfa *dfa)
size_t j, k;
for (j = i;
(BASE_TABLE(dfa)[j] & MATCH_FLAG_DIFF_ENCODE) &&
!(BASE_TABLE(dfa)[j] & MARK_DIFF_ENCODE);
((BASE_TABLE(dfa)[j] & MATCH_FLAG_DIFF_ENCODE) &&
!(BASE_TABLE(dfa)[j] & MARK_DIFF_ENCODE_VERIFIED));
j = k) {
if (BASE_TABLE(dfa)[j] & MARK_DIFF_ENCODE)
/* loop in current chain */
goto out;
k = DEFAULT_TABLE(dfa)[j];
if (j == k)
/* self loop */
goto out;
if (k < j)
break; /* already verified */
BASE_TABLE(dfa)[j] |= MARK_DIFF_ENCODE;
}
/* move mark to verified */
for (j = i;
(BASE_TABLE(dfa)[j] & MATCH_FLAG_DIFF_ENCODE);
j = k) {
k = DEFAULT_TABLE(dfa)[j];
if (j < i)
/* jumps to state/chain that has been
* verified
*/
break;
BASE_TABLE(dfa)[j] &= ~MARK_DIFF_ENCODE;
BASE_TABLE(dfa)[j] |= MARK_DIFF_ENCODE_VERIFIED;
}
}
error = 0;