On Thu, Apr 01, 2021 at 07:16:56AM +0200, Greg Kroah-Hartman wrote: > On Wed, Mar 31, 2021 at 07:21:45PM -0700, Kees Cook wrote: > > The sysfs interface to seq_file continues to be rather fragile > > (seq_get_buf() should not be used outside of seq_file), as seen with > > some recent exploits[1]. Move the seq_file buffer to the vmap area > > (while retaining the accounting flag), since it has guard pages that > > will catch and stop linear overflows. This seems justified given that > > sysfs's use of seq_file already uses kvmalloc(), is almost always using > > a PAGE_SIZE or larger allocation, has normally short-lived allocations, > > and is not normally on a performance critical path. > > > > Once seq_get_buf() has been removed (and all sysfs callbacks using > > seq_file directly), this change can also be removed. > > > > [1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html > > > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > > --- > > v3: > > - Limit to only sysfs (instead of all of seq_file). > > v2: https://lore.kernel.org/lkml/20210315174851.622228-1-keescook@xxxxxxxxxxxx/ > > v1: https://lore.kernel.org/lkml/20210312205558.2947488-1-keescook@xxxxxxxxxxxx/ > > --- > > fs/sysfs/file.c | 23 +++++++++++++++++++++++ > > 1 file changed, 23 insertions(+) > > > > diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c > > index 9aefa7779b29..70e7a450e5d1 100644 > > --- a/fs/sysfs/file.c > > +++ b/fs/sysfs/file.c > > @@ -16,6 +16,7 @@ > > #include <linux/mutex.h> > > #include <linux/seq_file.h> > > #include <linux/mm.h> > > +#include <linux/vmalloc.h> > > > > #include "sysfs.h" > > > > @@ -32,6 +33,25 @@ static const struct sysfs_ops *sysfs_file_ops(struct kernfs_node *kn) > > return kobj->ktype ? kobj->ktype->sysfs_ops : NULL; > > } > > > > +/* > > + * To be proactively defensive against sysfs show() handlers that do not > > + * correctly stay within their PAGE_SIZE buffer, use the vmap area to gain > > + * the trailing guard page which will stop linear buffer overflows. > > + */ > > +static void *sysfs_kf_seq_start(struct seq_file *sf, loff_t *ppos) > > +{ > > + struct kernfs_open_file *of = sf->private; > > + struct kernfs_node *kn = of->kn; > > + > > + WARN_ON_ONCE(sf->buf); > > How can buf ever not be NULL? And if it is, we will leak memory in the > next line so we shouldn't have _ONCE, we should always know, but not > rebooting the machine would be nice. It should never be possible. I did this because seq_file has some unusual buf allocation patterns in the kernel, and I liked the cheap leak check. I use _ONCE because spewing endlessly doesn't help most cases. And if you want to trigger it again, you don't have to reboot: https://www.kernel.org/doc/html/latest/admin-guide/clearing-warn-once.html > > > + sf->buf = __vmalloc(kn->attr.size, GFP_KERNEL_ACCOUNT); > > + if (!sf->buf) > > + return ERR_PTR(-ENOMEM); > > + sf->size = kn->attr.size; > > + > > + return NULL + !*ppos; > > +} > > Will this also cause the vmalloc fragmentation/abuse that others have > mentioned as userspace can trigger this? If I understood the concern correctly, it was about it being a risk for doing it for all seq_file uses. This version confines the changes to only sysfs seq_file uses. > And what code frees it? The existing hooks to seq_release() handle this already. This kind of "preallocation" of the seq_file buffer is done in a few places already (hence my desire for the sanity checking WARN lest future seq_file semantics change). -- Kees Cook
