From: Phillip Wood <phillip.wood@xxxxxxxxxxxxx> When dowild() cannot match a '*' or '/**/' wildcard then it must return WM_ABORT_TO_STARSTAR or WM_ABORT_ALL respectively. Failure to observe this results in unnecessary backtracking and the time taken for a failed match increases exponentially with the number of wildcards in the pattern [1]. Unfortunately in some instances dowild() returns WM_NOMATCH for a failed match resulting in long match times for patterns containing multiple wildcards as can be seen in the following benchmark. (Note that the timings in the Benchmark 1 are really measuring the time to execute test-tool rather than the time to match the pattern) Benchmark 1: t/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a" Time (mean ± σ): 22.8 ms ± 1.7 ms [User: 12.1 ms, System: 10.6 ms] Range (min … max): 19.4 ms … 26.9 ms 113 runs Warning: Ignoring non-zero exit code. Benchmark 2: t/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a*a*a*a*a*a*a*a*a" Time (mean ± σ): 5.244 s ± 0.228 s [User: 5.229 s, System: 0.010 s] Range (min … max): 4.969 s … 5.707 s 10 runs Warning: Ignoring non-zero exit code. Summary 't/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a"' ran 230.37 ± 20.04 times faster than 't/helper/test-tool wildmatch wildmatch aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab "*a*a*a*a*a*a*a*a*a"' The security implications are limited as it only affects operations that are potentially DoS vectors. For example by creating a blob containing such a pattern a malicious user can exploit this behavior to use large amounts of CPU time on a remote server by pushing the blob and then creating a new clone with --filter=sparse:oid. However this filter type is usually disabled as it is known to consume large amounts of CPU time even without this bug. The WM_MATCH changed in the first hunk of this patch comes from the original implementation imported from rsync in 5230f605e1 (Import wildmatch from rsync, 2012-10-15). Compared to the others converted here it is fairly harmless as it only triggers at the end of the pattern and so will only cause a single unnecessary backtrack. The others introduced by 6f1a31f0aa (wildmatch: advance faster in <asterisk> + <literal> patterns, 2013-01-01) and 46983441ae (wildmatch: make a special case for "*/" with FNM_PATHNAME, 2013-01-01) are more pernicious and will cause exponential behavior. A new test is added to protect against future regressions. [1] https://research.swtch.com/glob Helped-by: Derrick Stolee <derrickstolee@xxxxxxxxxx> Signed-off-by: Phillip Wood <phillip.wood@xxxxxxxxxxxxx> --- t/t3070-wildmatch.sh | 9 +++++++++ wildmatch.c | 12 ++++++++---- 2 files changed, 17 insertions(+), 4 deletions(-) diff --git a/t/t3070-wildmatch.sh b/t/t3070-wildmatch.sh index 5d871fde96..b91a7cb712 100755 --- a/t/t3070-wildmatch.sh +++ b/t/t3070-wildmatch.sh @@ -431,4 +431,13 @@ match 1 1 1 1 'a' '[B-a]' match 0 1 0 1 'z' '[Z-y]' match 1 1 1 1 'Z' '[Z-y]' +test_expect_success 'matching does not exhibit exponential behavior' ' + test-tool wildmatch wildmatch \ + aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaab \ + "*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a*a" & + pid=$! && + sleep 2 && + ! kill $! +' + test_done diff --git a/wildmatch.c b/wildmatch.c index 7e5a7ea1ea..06861bd8bc 100644 --- a/wildmatch.c +++ b/wildmatch.c @@ -114,7 +114,7 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags) * only if there are no more slash characters. */ if (!match_slash) { if (strchr((char *)text, '/')) - return WM_NOMATCH; + return WM_ABORT_TO_STARSTAR; } return WM_MATCH; } else if (!match_slash && *p == '/') { @@ -125,7 +125,7 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags) */ const char *slash = strchr((char*)text, '/'); if (!slash) - return WM_NOMATCH; + return WM_ABORT_ALL; text = (const uchar*)slash; /* the slash is consumed by the top-level for loop */ break; @@ -153,8 +153,12 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags) break; text++; } - if (t_ch != p_ch) - return WM_NOMATCH; + if (t_ch != p_ch) { + if (match_slash) + return WM_ABORT_ALL; + else + return WM_ABORT_TO_STARSTAR; + } } if ((matched = dowild(p, text, flags)) != WM_NOMATCH) { if (!match_slash || matched != WM_ABORT_TO_STARSTAR) -- 2.39.2
