attr: fix msan issue in read_attr_from_index

From: Kyle Lippincott <[email protected]>

Memory sanitizer (msan) is detecting a use of an uninitialized variable (size) in read_attr_from_index:

    ==2268==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x5651f3416504 in read_attr_from_index git/attr.c:868:11
    #1 0x5651f3415530 in read_attr git/attr.c
    #2 0x5651f3413d74 in bootstrap_attr_stack git/attr.c:968:6
    #3 0x5651f3413d74 in prepare_attr_stack git/attr.c:1004:2
    #4 0x5651f3413d74 in collect_some_attrs git/attr.c:1199:2
    #5 0x5651f3413144 in git_check_attr git/attr.c:1345:2
    #6 0x5651f34728da in convert_attrs git/convert.c:1320:2
    #7 0x5651f3473425 in would_convert_to_git_filter_fd git/convert.c:1373:2
    #8 0x5651f357a35e in index_fd git/object-file.c:2630:34
    #9 0x5651f357aa15 in index_path git/object-file.c:2657:7
    #10 0x5651f35db9d9 in add_to_index git/read-cache.c:766:7
    #11 0x5651f35dc170 in add_file_to_index git/read-cache.c:799:9
    #12 0x5651f321f9b2 in add_files git/builtin/add.c:346:7
    #13 0x5651f321f9b2 in cmd_add git/builtin/add.c:565:18
    #14 0x5651f321d327 in run_builtin git/git.c:474:11
    #15 0x5651f321bc9e in handle_builtin git/git.c:729:3
    #16 0x5651f321a792 in run_argv git/git.c:793:4
    #17 0x5651f321a792 in cmd_main git/git.c:928:19
    #18 0x5651f33dde1f in main git/common-main.c:62:11

The issue exists because size is an output parameter from read_blob_data_from_index, but it's only modified if read_blob_data_from_index returns non-NULL. The read of size when calling read_attr_from_buf unconditionally may read from an uninitialized value. read_attr_from_buf checks that buf is non-NULL before reading from size, but by then it's already too late: the uninitialized read will have happened already. Furthermore, there's no guarantee that the compiler won't reorder things so that it checks size before checking !buf.

Make the call to read_attr_from_buf conditional on buf being non-NULL, ensuring that size is not read if it's never set.

---
attr: fix msan issue in read_attr_from_index

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1747%2Fspectral54%2Fmsan-attr-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1747/spectral54/msan-attr-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1747

 attr.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index 300f994ba6e..a2e0775f7e5 100644
@@ -865,7 +865,8 @@ static struct attr_stack *read_attr_from_index(struct index_state *istate,
 		stack = read_attr_from_blob(istate, &istate->cache[sparse_dir_pos]->oid, relative_path, flags);
 	} else {
 		buf = read_blob_data_from_index(istate, path, &size);
-		stack = read_attr_from_buf(buf, size, path, flags);
+		if (buf)
+			stack = read_attr_from_buf(buf, size, path, flags);
 	}
 	return stack;
 }

base-commit: 8d94cfb54504f2ec9edc7ca3eb5c29a3dd3675ae

gitgitgadget

Makes good sense.

Correct.

Yes, but it is dubious that reading an uninitialized value that we know will not be used is a problem, so I am inclined to say that MSAN is giving a false positive here.

This I do not understand. Are you talking about buf vs length here in the callee?

        static struct attr_stack *read_attr_from_buf(char *buf, size_t length,
                                                     const char *path, unsigned flags)
        {
                struct attr_stack *res;
                char *sp;
                int lineno = 0;

                if (!buf)
                        return NULL;
                if (length >= ATTR_MAX_FILE_SIZE) {
                        warning(_("ignoring overly large gitattributes blob '%s'"), path);
                        free(buf);
                        return NULL;
                }

At the machine level, a prefetch may happen from both buf and length, but the program ought to behave the same way as the code is executed serially as written. If the compiler allows the outside world to observe that resulting code checks length even when buf is NULL, such a compiler is broken. So I do not think that is what you are referring to, but then I do not know what problem you are describing.

Having said all that ...

... this makes the logic at the caller crystal clear, so even if there are suboptimal checker that bothers us with false positives, the change itself justifies itself, I would say.

Well, "even if there were *no* MSAN or other issues wrt usage of size" was what I wanted to say. Sorry for a noise.

Once there's an uninitialized read, we're in undefined behavior territory. There's no requirement that the compiler keep the code operating the way we'd logically expect once there's undefined behavior, especially with the optimizer involved.

I think that you're right though: when I wrote this I'd convinced myself that this wasn't guaranteed to work, but taking a look now I can't think of a way for this to go wrong, because afaik size_t is such a simple type, conceptually. When compiling read_attr_from_buf, it can't actually assume any relationship between buf and length. Maybe I was thinking that with link-time optimizations (whole-program optimizations) it can go wrong? I'm not remembering what I was thinking about when I wrote that, sorry.

Yeah, this is the same one I mentioned when bisecting in the other thread[1]. But I got confused by applying my fixup patch at various points in the bisection, and thought it _used_ to be a problem, and isn't anymore. It's the other way around. It was introduced by c793f9cb08, which moved the NULL check into the helper.

That patch is from Taylor, but I'm listed as a co-author, and I'm almost certain moving that NULL check was my suggestion. So it's doubly bad that I didn't figure out what was going on earlier. ;)

Possible UB aside, I doubt this can trigger bad behavior in practice. But I also wouldn't call it a false positive in MSan. We really are reading the uninitialized value and passing it. Your fix here is the obviously correct thing to do.

-Peff

[1] https://lore.kernel.org/git/[email protected]/