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Fri, 06 Nov 2020 09:02:25 -0800 (PST) From: Jinoh Kang Subject: [PATCH v2] diff: handle NULL filespecs in run_external_diff To: Junio C Hamano , git@vger.kernel.org Cc: Johannes Schindelin References: Message-ID: Date: Fri, 6 Nov 2020 17:02:19 +0000 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Firefox/78.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org [forwarding from git-security@googlegroups.com to git@vger.kernel.org] On 11/4/20 10:30 PM, Junio C Hamano wrote: > Jinoh Kang writes: > >> `one` and `two` can be NULL when comparing an unmerged pair. >> There is a conditional above that already tests for this. >> >> Fixes: 3aef54e8b8 ("diff: munmap() file contents before running external diff") >> Signed-off-by: Jinoh Kang >> --- >> diff.c | 6 ++++-- >> t/t7800-difftool.sh | 23 +++++++++++++++++++++++ >> 2 files changed, 27 insertions(+), 2 deletions(-) >> >> diff --git a/diff.c b/diff.c >> index d24f47df99..ae1ec2d6c8 100644 >> --- a/diff.c >> +++ b/diff.c >> @@ -4267,8 +4267,10 @@ static void run_external_diff(const char *pgm, >> strvec_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter); >> strvec_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr); >> >> - diff_free_filespec_data(one); >> - diff_free_filespec_data(two); >> + if (one) >> + diff_free_filespec_data(one); >> + if (two) >> + diff_free_filespec_data(two); >> if (run_command_v_opt_cd_env(argv.v, RUN_USING_SHELL, NULL, env.v)) >> die(_("external diff died, stopping at %s"), name); > > Have you considered allowing diff_free_filespec_data() to take NULL > and return safely without doing anything? Yes, I have. In fact, this kind of behavior is exactly what I would expect from a function that "frees" something. However, I was not entirely sure if this applies here, for several reasons that follow: > That models after free() and other "we are done with the resource > and it is time to clean it up" functions, This corresponds to `free_filespec`. In this particular case, I strongly agree that it makes perfect sense to do nothing when passed a NULL pointer. However, I humbly opine that the free() semantics do not apply to `diff_free_filespec_data`; rather, I prefer to see it as a function that simply transitions a diff_filespec from one state to another. For this reason, I'd consider the act of passing NULL to diff_free_filespec_data as a bug in the first place. Further, if it does not entail a security issue, why not just crash *right now* rather than (possibly) causing more obscure bugs later? I would put the blame on its name, since "data" feels too generic and makes the function sound like freeing the filespec _itself_. diff_filespec carries a lot of other things besides just `data` and `cnt_data`. Please feel free to correct me if I'm wrong; after all, I am not exactly one of the long-time maintainers. > fixes this particular bug, and possibly simplifies existing > callers that check the NULL-ness before calling it. I was unable to find any callsites that explicitly check for NULL-ness _immediately_ before calling diff_free_filespec_data. Excluding 3aef54e8b8, `GCC -fanalyze` suggests to me that all the callers have already dereferenced the pointer by the time the call is made; therefore, a NULL pointer dereference would have already happened before the flow could even get to the diff_free_filespec_data function. The NULL checks are usually placed in the beginning of caller functions rather than close to the exit path (which then calls diff_free_filespec_data). > >> diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh >> index 524f30f7dc..8cc1c9117c 100755 >> --- a/t/t7800-difftool.sh >> +++ b/t/t7800-difftool.sh >> @@ -728,6 +728,29 @@ test_expect_success 'add -N and difftool -d' ' >> git difftool --dir-diff --extcmd ls >> ' >> >> +test_expect_success 'difftool --cached with unmerged files' ' >> + test_when_finished git reset --hard && >> + echo base > file && > > Style. "echo base >file &&" (no SP between redirect operator and > its target). My bad. Thanks! > > I do not think this is "security" matter. I do appreciate you > erring on the side of being cautious and sending the patch first > here, but please take it to the regular mailing list. Yes, no sensible web-based git browser would prepare a working tree, start a conflicting merge *AND* run an external diff on it... > > Thanks. > -- Jinoh Kang Theori