From ba07d31bd2140190c4d8c197c9b8a503544b4c29 Mon Sep 17 00:00:00 2001 From: Minjae Kim Date: Sat, 27 Mar 2021 14:05:56 +0900 Subject: [PATCH] checkout: fix bug that makes checkout follow symlinks in leading path Before checking out a file, we have to confirm that all of its leading components are real existing directories. And to reduce the number of lstat() calls in this process, we cache the last leading path known to contain only directories. However, when a path collision occurs (e.g. when checking out case-sensitive files in case-insensitive file systems), a cached path might have its file type changed on disk, leaving the cache on an invalid state. Normally, this doesn't bring any bad consequences as we usually check out files in index order, and therefore, by the time the cached path becomes outdated, we no longer need it anyway (because all files in that directory would have already been written). But, there are some users of the checkout machinery that do not always follow the index order. In particular: checkout-index writes the paths in the same order that they appear on the CLI (or stdin); and the delayed checkout feature -- used when a long-running filter process replies with "status=delayed" -- postpones the checkout of some entries, thus modifying the checkout order. When we have to check out an out-of-order entry and the lstat() cache is invalid (due to a previous path collision), checkout_entry() may end up using the invalid data and thrusting that the leading components are real directories when, in reality, they are not. In the best case scenario, where the directory was replaced by a regular file, the user will get an error: "fatal: unable to create file 'foo/bar': Not a directory". But if the directory was replaced by a symlink, checkout could actually end up following the symlink and writing the file at a wrong place, even outside the repository. Since delayed checkout is affected by this bug, it could be used by an attacker to write arbitrary files during the clone of a maliciously crafted repository. Some candidate solutions considered were to disable the lstat() cache during unordered checkouts or sort the entries before passing them to the checkout machinery. But both ideas include some performance penalty and they don't future-proof the code against new unordered use cases. Instead, we now manually reset the lstat cache whenever we successfully remove a directory. Note: We are not even checking whether the directory was the same as the lstat cache points to because we might face a scenario where the paths refer to the same location but differ due to case folding, precomposed UTF-8 issues, or the presence of `..` components in the path. Two regression tests, with case-collisions and utf8-collisions, are also added for both checkout-index and delayed checkout. Note: to make the previously mentioned clone attack unfeasible, it would be sufficient to reset the lstat cache only after the remove_subtree() call inside checkout_entry(). This is the place where we would remove a directory whose path collides with the path of another entry that we are currently trying to check out (possibly a symlink). However, in the interest of a thorough fix that does not leave Git open to similar-but-not-identical attack vectors, we decided to intercept all `rmdir()` calls in one fell swoop. This addresses CVE-2021-21300. Co-authored-by: Johannes Schindelin Signed-off-by: Matheus Tavares Upstream-Status: Acepted [https://github.com/git/git/commit/684dd4c2b414bcf648505e74498a608f28de4592] CVE: CVE-2021-21300 Signed-off-by: Minjae Kim --- cache.h | 1 + compat/mingw.c | 2 ++ git-compat-util.h | 5 +++++ symlinks.c | 24 ++++++++++++++++++++ t/t0021-conversion.sh | 39 ++++++++++++++++++++++++++++++++ t/t0021/rot13-filter.pl | 21 ++++++++++++++--- t/t2006-checkout-index-basic.sh | 40 +++++++++++++++++++++++++++++++++ 7 files changed, 129 insertions(+), 3 deletions(-) diff --git a/cache.h b/cache.h index 6544264..64226c3 100644 --- a/cache.h +++ b/cache.h @@ -1733,6 +1733,7 @@ int has_symlink_leading_path(const char *name, int len); int threaded_has_symlink_leading_path(struct cache_def *, const char *, int); int check_leading_path(const char *name, int len); int has_dirs_only_path(const char *name, int len, int prefix_len); +extern void invalidate_lstat_cache(void); void schedule_dir_for_removal(const char *name, int len); void remove_scheduled_dirs(void); diff --git a/compat/mingw.c b/compat/mingw.c index 8ee0b64..be2b88e 100644 --- a/compat/mingw.c +++ b/compat/mingw.c @@ -364,6 +364,8 @@ int mingw_rmdir(const char *pathname) ask_yes_no_if_possible("Deletion of directory '%s' failed. " "Should I try again?", pathname)) ret = _wrmdir(wpathname); + if (!ret) + invalidate_lstat_cache(); return ret; } diff --git a/git-compat-util.h b/git-compat-util.h index 5637114..d983853 100644 --- a/git-compat-util.h +++ b/git-compat-util.h @@ -345,6 +345,11 @@ static inline int noop_core_config(const char *var, const char *value, void *cb) #define platform_core_config noop_core_config #endif +int lstat_cache_aware_rmdir(const char *path); +#if !defined(__MINGW32__) && !defined(_MSC_VER) +#define rmdir lstat_cache_aware_rmdir +#endif + #ifndef has_dos_drive_prefix static inline int git_has_dos_drive_prefix(const char *path) { diff --git a/symlinks.c b/symlinks.c index 69d458a..7dbb6b2 100644 --- a/symlinks.c +++ b/symlinks.c @@ -267,6 +267,13 @@ int has_dirs_only_path(const char *name, int len, int prefix_len) */ static int threaded_has_dirs_only_path(struct cache_def *cache, const char *name, int len, int prefix_len) { + /* + * Note: this function is used by the checkout machinery, which also + * takes care to properly reset the cache when it performs an operation + * that would leave the cache outdated. If this function starts caching + * anything else besides FL_DIR, remember to also invalidate the cache + * when creating or deleting paths that might be in the cache. + */ return lstat_cache(cache, name, len, FL_DIR|FL_FULLPATH, prefix_len) & FL_DIR; @@ -321,3 +328,20 @@ void remove_scheduled_dirs(void) { do_remove_scheduled_dirs(0); } + +void invalidate_lstat_cache(void) +{ + reset_lstat_cache(&default_cache); +} + +#undef rmdir +int lstat_cache_aware_rmdir(const char *path) +{ + /* Any change in this function must be made also in `mingw_rmdir()` */ + int ret = rmdir(path); + + if (!ret) + invalidate_lstat_cache(); + + return ret; +} diff --git a/t/t0021-conversion.sh b/t/t0021-conversion.sh index 4bfffa9..c42f51e 100755 --- a/t/t0021-conversion.sh +++ b/t/t0021-conversion.sh @@ -957,4 +957,43 @@ test_expect_success PERL 'invalid file in delayed checkout' ' grep "error: external filter .* signaled that .unfiltered. is now available although it has not been delayed earlier" git-stderr.log ' +for mode in 'case' 'utf-8' +do + case "$mode" in + case) dir='A' symlink='a' mode_prereq='CASE_INSENSITIVE_FS' ;; + utf-8) + dir=$(printf "\141\314\210") symlink=$(printf "\303\244") + mode_prereq='UTF8_NFD_TO_NFC' ;; + esac + + test_expect_success PERL,SYMLINKS,$mode_prereq \ + "delayed checkout with $mode-collision don't write to the wrong place" ' + test_config_global filter.delay.process \ + "\"$TEST_ROOT/rot13-filter.pl\" --always-delay delayed.log clean smudge delay" && + test_config_global filter.delay.required true && + git init $mode-collision && + ( + cd $mode-collision && + mkdir target-dir && + empty_oid=$(printf "" | git hash-object -w --stdin) && + symlink_oid=$(printf "%s" "$PWD/target-dir" | git hash-object -w --stdin) && + attr_oid=$(echo "$dir/z filter=delay" | git hash-object -w --stdin) && + cat >objs <<-EOF && + 100644 blob $empty_oid $dir/x + 100644 blob $empty_oid $dir/y + 100644 blob $empty_oid $dir/z + 120000 blob $symlink_oid $symlink + 100644 blob $attr_oid .gitattributes + EOF + git update-index --index-info +# +# Log path defines a debug log file that the script writes to. The +# subsequent arguments define a list of supported protocol capabilities +# ("clean", "smudge", etc). +# +# When --always-delay is given all pathnames with the "can-delay" flag +# that don't appear on the list bellow are delayed with a count of 1 +# (see more below). # # This implementation supports special test cases: # (1) If data with the pathname "clean-write-fail.r" is processed with @@ -53,6 +59,13 @@ sub gitperllib { use Git::Packet; my $MAX_PACKET_CONTENT_SIZE = 65516; + +my $always_delay = 0; +if ( $ARGV[0] eq '--always-delay' ) { + $always_delay = 1; + shift @ARGV; +} + my $log_file = shift @ARGV; my @capabilities = @ARGV; @@ -134,6 +147,8 @@ sub rot13 { if ( $buffer eq "can-delay=1" ) { if ( exists $DELAY{$pathname} and $DELAY{$pathname}{"requested"} == 0 ) { $DELAY{$pathname}{"requested"} = 1; + } elsif ( !exists $DELAY{$pathname} and $always_delay ) { + $DELAY{$pathname} = { "requested" => 1, "count" => 1 }; } } elsif ($buffer =~ /^(ref|treeish|blob)=/) { print $debug " $buffer"; diff --git a/t/t2006-checkout-index-basic.sh b/t/t2006-checkout-index-basic.sh index 57cbdfe..f223a02 100755 --- a/t/t2006-checkout-index-basic.sh +++ b/t/t2006-checkout-index-basic.sh @@ -21,4 +21,44 @@ test_expect_success 'checkout-index -h in broken repository' ' test_i18ngrep "[Uu]sage" broken/usage ' +for mode in 'case' 'utf-8' +do + case "$mode" in + case) dir='A' symlink='a' mode_prereq='CASE_INSENSITIVE_FS' ;; + utf-8) + dir=$(printf "\141\314\210") symlink=$(printf "\303\244") + mode_prereq='UTF8_NFD_TO_NFC' ;; + esac + + test_expect_success SYMLINKS,$mode_prereq \ + "checkout-index with $mode-collision don't write to the wrong place" ' + git init $mode-collision && + ( + cd $mode-collision && + mkdir target-dir && + empty_obj_hex=$(git hash-object -w --stdin objs <<-EOF && + 100644 blob ${empty_obj_hex} ${dir}/x + 100644 blob ${empty_obj_hex} ${dir}/y + 100644 blob ${empty_obj_hex} ${dir}/z + 120000 blob ${symlink_hex} ${symlink} + EOF + git update-index --index-info