Re: Crash in VirtualBox virtual machines running kernel 6.5

[Kees Cook <keescook@chromium.org>]

On Wed, Jul 19, 2023 at 10:02:36AM -0500, Larry Finger wrote:
> Hi,
> 
> When I try to start a VirtualBox virtual machine running kernel 6.5-rc2, it
> gets a kernel bug as follows while trying to mount a vboxsf-shared mount:
> 
> Jul 19 08:48:19 localhost kernel: detected buffer overflow in strscpy
> Jul 19 08:48:19 localhost kernel: ------------[ cut here ]------------
> Jul 19 08:48:19 localhost kernel: kernel BUG at lib/string_helpers.c:1031!
> [...]
> Jul 19 08:48:19 localhost kernel: Call Trace:
> [...]
> Jul 19 08:48:19 localhost kernel:  vboxsf_fill_super+0x3bc/0x3c0 [vboxsf 447dff7257fbc53f0b47ed873d2b02eb4773401c]
> [...]
> 
> The traceback points to the strscpy() added in commit 883f8fe87686d, which
> ironically was submitted to avoid buffer overflows using strlcpy(); however,
> I do not think that is the problem. My suspicion is that it comes from
> struct shfl_string, and the definition of the variable-length arrays in the
> union, and that their lengths are confusing the kernel's string handling
> routines.

Ah, hm, I think this may still warn with 883f8fe87686d reverted, as it
seems the issue is the fake flexible arrays in struct shfl_string. Likely
the patch manifesting the false positive is df8fc4e934c1 ("kbuild:
Enable -fstrict-flex-arrays=3"), if you're building with GCC 13.

> I will be happy to test any proposed patches.

Thank! Can you see if this fixes it?

diff --git a/fs/vboxsf/shfl_hostintf.h b/fs/vboxsf/shfl_hostintf.h
index aca829062c12..243d1b91bb45 100644
--- a/fs/vboxsf/shfl_hostintf.h
+++ b/fs/vboxsf/shfl_hostintf.h
@@ -68,9 +68,8 @@ struct shfl_string {
 
 	/** UTF-8 or UTF-16 string. Nul terminated. */
 	union {
-		u8 utf8[2];
-		u16 utf16[1];
-		u16 ucs2[1]; /* misnomer, use utf16. */
+		DECLARE_FLEX_ARRAY(u8, utf8);
+		DECLARE_FLEX_ARRAY(u16, utf16);
 	} string;
 };
 VMMDEV_ASSERT_SIZE(shfl_string, 6);


(I note that "ucs" is used in the kernel source, and contains a comment
that it shouldn't be used, so I removed it.)

-- 
Kees Cook