tjh.dev / kernel
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apparmor: fix differential encoding verification

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: 031dcc8f4e84 ("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>

John Johansen 39440b13 6601e13e

+20 -4
+1
security/apparmor/include/match.h
··· 185 185 #define MATCH_FLAG_DIFF_ENCODE 0x80000000 186 186 #define MARK_DIFF_ENCODE 0x40000000 187 187 #define MATCH_FLAG_OOB_TRANSITION 0x20000000 188 + #define MARK_DIFF_ENCODE_VERIFIED 0x10000000 188 189 #define MATCH_FLAGS_MASK 0xff000000 189 190 #define MATCH_FLAGS_VALID (MATCH_FLAG_DIFF_ENCODE | MATCH_FLAG_OOB_TRANSITION) 190 191 #define MATCH_FLAGS_INVALID (MATCH_FLAGS_MASK & ~MATCH_FLAGS_VALID)
+19 -4
security/apparmor/match.c
··· 202 202 size_t j, k; 203 203 204 204 for (j = i; 205 - (BASE_TABLE(dfa)[j] & MATCH_FLAG_DIFF_ENCODE) && 206 - !(BASE_TABLE(dfa)[j] & MARK_DIFF_ENCODE); 205 + ((BASE_TABLE(dfa)[j] & MATCH_FLAG_DIFF_ENCODE) && 206 + !(BASE_TABLE(dfa)[j] & MARK_DIFF_ENCODE_VERIFIED)); 207 207 j = k) { 208 + if (BASE_TABLE(dfa)[j] & MARK_DIFF_ENCODE) 209 + /* loop in current chain */ 210 + goto out; 208 211 k = DEFAULT_TABLE(dfa)[j]; 209 212 if (j == k) 213 + /* self loop */ 210 214 goto out; 211 - if (k < j) 212 - break; /* already verified */ 213 215 BASE_TABLE(dfa)[j] |= MARK_DIFF_ENCODE; 216 + } 217 + /* move mark to verified */ 218 + for (j = i; 219 + (BASE_TABLE(dfa)[j] & MATCH_FLAG_DIFF_ENCODE); 220 + j = k) { 221 + k = DEFAULT_TABLE(dfa)[j]; 222 + if (j < i) 223 + /* jumps to state/chain that has been 224 + * verified 225 + */ 226 + break; 227 + BASE_TABLE(dfa)[j] &= ~MARK_DIFF_ENCODE; 228 + BASE_TABLE(dfa)[j] |= MARK_DIFF_ENCODE_VERIFIED; 214 229 } 215 230 } 216 231 error = 0;