Messages in this thread Patch in this message | | | Date | Fri, 28 Oct 2022 15:56:37 -0700 | From | Kees Cook <> | Subject | Re: [PATCH] jfs: Fix fortify moan in symlink |
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On Mon, Oct 24, 2022 at 07:49:17PM +0100, Dr. David Alan Gilbert wrote: > * Kees Cook (keescook@chromium.org) wrote: > > On Sat, Oct 22, 2022 at 09:39:14PM +0100, linux@treblig.org wrote: > > > From: "Dr. David Alan Gilbert" <linux@treblig.org> > > > > > > JFS has in jfs_incore.h: > > > > > > /* _inline may overflow into _inline_ea when needed */ > > > /* _inline_ea may overlay the last part of > > > * file._xtroot if maxentry = XTROOTINITSLOT > > > */ > > > union { > > > struct { > > > /* 128: inline symlink */ > > > unchar _inline[128]; > > > /* 128: inline extended attr */ > > > unchar _inline_ea[128]; > > > }; > > > unchar _inline_all[256]; > > > > > > and currently the symlink code copies into _inline; > > > if this is larger than 128 bytes it triggers a fortify warning of the > > > form: > > > > > > memcpy: detected field-spanning write (size 132) of single field > > > "ip->i_link" at fs/jfs/namei.c:950 (size 18446744073709551615) > > > > Which compiler are you using for this build? > > I think that report was the same on gcc on Fedora 37 and whatever > syzkaller was running. > > > This size report (SIZE_MAX) > > should be impossible to reach. But also, the size is just wrong -- > > i_inline is 128 bytes, not SIZE_MAX. So, the detection is working > > (132 > 128), but the report is broken, and I can't see how... > > Yeh, and led me down a blind alley for a while thinking something had > really managed to screwup the strlen somehow.
This looks like a GCC bug (going at least back to GCC 10.2)[1], but some extra care around the macro appears to make it go away, so the reporting variable doesn't get confused/re-evaluated:
diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h index 09a032f6ce6b..9e2d96993c30 100644 --- a/include/linux/fortify-string.h +++ b/include/linux/fortify-string.h @@ -550,13 +550,18 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size, #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \ p_size_field, q_size_field, op) ({ \ - size_t __fortify_size = (size_t)(size); \ - WARN_ONCE(fortify_memcpy_chk(__fortify_size, p_size, q_size, \ - p_size_field, q_size_field, #op), \ + const size_t __fortify_size = (size_t)(size); \ + const size_t __p_size = (p_size); \ + const size_t __q_size = (q_size); \ + const size_t __p_size_field = (p_size_field); \ + const size_t __q_size_field = (q_size_field); \ + WARN_ONCE(fortify_memcpy_chk(__fortify_size, __p_size, \ + __q_size, __p_size_field, \ + __q_size_field, #op), \ #op ": detected field-spanning write (size %zu) of single %s (size %zu)\n", \ __fortify_size, \ "field \"" #p "\" at " __FILE__ ":" __stringify(__LINE__), \ - p_size_field); \ + __p_size_field); \ __underlying_##op(p, q, __fortify_size); \ })
[1] https://syzkaller.appspot.com/bug?id=23d613df5259b977dac1696bec77f61a85890e3d
-- Kees Cook
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