| Message ID | 20250204213207.337980-1-mjguzik@gmail.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | vfs: sanity check the length passed to inode_set_cached_link() | expand |
On Tue, Feb 4, 2025 at 10:32 PM Mateusz Guzik <mjguzik@gmail.com> wrote: > > This costs a strlen() call when instatianating a symlink. > > Preferably it would be hidden behind VFS_WARN_ON (or compatible), but > there is no such facility at the moment. With the facility in place the > call can be patched out in production kernels. > > In the meantime, since the cost is being paid unconditionally, use the > result to a fixup the bad caller. > > This is not expected to persist in the long run (tm). > > Sample splat: > bad length passed for symlink [/tmp/syz-imagegen43743633/file0/file0] (got 131109, expected 37) > [rest of WARN blurp goes here] > > Signed-off-by: Mateusz Guzik <mjguzik@gmail.com> > --- > > This has a side effect of working around the panic reported in: > https://lore.kernel.org/all/67a1e1f4.050a0220.163cdc.0063.GAE@google.com/ > > I'm confident this merely exposed a bug in ext4, see: > https://lore.kernel.org/all/CAGudoHEv+Diti3r0x9VmF5ixgRVKk4trYnX_skVJNkQoTMaDHg@mail.gmail.com/#t > > Nonethelss, should help catch future problems. > > include/linux/fs.h | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/include/linux/fs.h b/include/linux/fs.h > index be3ad155ec9f..1437a3323731 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -791,6 +791,19 @@ struct inode { > > static inline void inode_set_cached_link(struct inode *inode, char *link, int linklen) > { > + int testlen; > + > + /* > + * TODO: patch it into a debug-only check if relevant macros show up. > + * In the meantime, since we are suffering strlen even on production kernels > + * to find the right length, do a fixup if the wrong value got passed. > + */ > + testlen = strlen(link); > + if (testlen != linklen) { > + WARN_ONCE(1, "bad length passed for symlink [%s] (got %d, expected %d)", > + link, linklen, testlen); I just remembered that path_noexec thing got a CVE. I suspect someone may try to assign one here for user-mountable disk images triggering this warn. Can be patched to "printk_once", but then it does not implicitly tell which fs is messing up due to lack of a backtrace. > + linklen = testlen; > + } > inode->i_link = link; > inode->i_linklen = linklen; > inode->i_opflags |= IOP_CACHED_LINK; > -- > 2.43.0 >
On Tue, Feb 04, 2025 at 10:32:07PM +0100, Mateusz Guzik wrote: > This costs a strlen() call when instatianating a symlink. > > Preferably it would be hidden behind VFS_WARN_ON (or compatible), but > there is no such facility at the moment. With the facility in place the > call can be patched out in production kernels. > > In the meantime, since the cost is being paid unconditionally, use the > result to a fixup the bad caller. > > This is not expected to persist in the long run (tm). > > Sample splat: > bad length passed for symlink [/tmp/syz-imagegen43743633/file0/file0] (got 131109, expected 37) > [rest of WARN blurp goes here] > > Signed-off-by: Mateusz Guzik <mjguzik@gmail.com> > --- > > This has a side effect of working around the panic reported in: > https://lore.kernel.org/all/67a1e1f4.050a0220.163cdc.0063.GAE@google.com/ > > I'm confident this merely exposed a bug in ext4, see: > https://lore.kernel.org/all/CAGudoHEv+Diti3r0x9VmF5ixgRVKk4trYnX_skVJNkQoTMaDHg@mail.gmail.com/#t Yes, ext4 should not continue to load a fast symlink where i_size > sizeof(i_data). I'm a little surprised that nd_terminate_link exists to paper over this situation, but the commit adding it (035146851cfa2fe) is from 2008 when we were all young and naïve. But yes, would be nice to have a CONFIG_VFS_DEBUG behind which to hide assertions so that insufficient validation in ext4 will burble up through Ted's test infrastructure. ;) That said, I just saw this in ext4_get_link: nd_terminate_link(bh->b_data, inode->i_size, inode->i_sb->s_blocksize - 1); Which is writing into a buffer head without a transaction? Granted symlink targets in ext4 are written out to disk with the null terminator so I guess that's not a big deal. --D > > Nonethelss, should help catch future problems. > > include/linux/fs.h | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/include/linux/fs.h b/include/linux/fs.h > index be3ad155ec9f..1437a3323731 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -791,6 +791,19 @@ struct inode { > > static inline void inode_set_cached_link(struct inode *inode, char *link, int linklen) > { > + int testlen; > + > + /* > + * TODO: patch it into a debug-only check if relevant macros show up. > + * In the meantime, since we are suffering strlen even on production kernels > + * to find the right length, do a fixup if the wrong value got passed. > + */ > + testlen = strlen(link); > + if (testlen != linklen) { > + WARN_ONCE(1, "bad length passed for symlink [%s] (got %d, expected %d)", > + link, linklen, testlen); > + linklen = testlen; > + } > inode->i_link = link; > inode->i_linklen = linklen; > inode->i_opflags |= IOP_CACHED_LINK; > -- > 2.43.0 > >
diff --git a/include/linux/fs.h b/include/linux/fs.h index be3ad155ec9f..1437a3323731 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -791,6 +791,19 @@ struct inode { static inline void inode_set_cached_link(struct inode *inode, char *link, int linklen) { + int testlen; + + /* + * TODO: patch it into a debug-only check if relevant macros show up. + * In the meantime, since we are suffering strlen even on production kernels + * to find the right length, do a fixup if the wrong value got passed. + */ + testlen = strlen(link); + if (testlen != linklen) { + WARN_ONCE(1, "bad length passed for symlink [%s] (got %d, expected %d)", + link, linklen, testlen); + linklen = testlen; + } inode->i_link = link; inode->i_linklen = linklen; inode->i_opflags |= IOP_CACHED_LINK;
This costs a strlen() call when instatianating a symlink. Preferably it would be hidden behind VFS_WARN_ON (or compatible), but there is no such facility at the moment. With the facility in place the call can be patched out in production kernels. In the meantime, since the cost is being paid unconditionally, use the result to a fixup the bad caller. This is not expected to persist in the long run (tm). Sample splat: bad length passed for symlink [/tmp/syz-imagegen43743633/file0/file0] (got 131109, expected 37) [rest of WARN blurp goes here] Signed-off-by: Mateusz Guzik <mjguzik@gmail.com> --- This has a side effect of working around the panic reported in: https://lore.kernel.org/all/67a1e1f4.050a0220.163cdc.0063.GAE@google.com/ I'm confident this merely exposed a bug in ext4, see: https://lore.kernel.org/all/CAGudoHEv+Diti3r0x9VmF5ixgRVKk4trYnX_skVJNkQoTMaDHg@mail.gmail.com/#t Nonethelss, should help catch future problems. include/linux/fs.h | 13 +++++++++++++ 1 file changed, 13 insertions(+)