On Thu, Apr 01, 2021 at 09:10:05AM +0200, Greg Kroah-Hartman wrote: > On Wed, Mar 31, 2021 at 11:52:20PM -0700, Kees Cook wrote: > > 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 > > True, I was thinking of the panic-on-warn people, and the hesitation of > adding new WARN_ON() to the kernel code. If this really can happen, > shouldn't we handle it properly? It should never happen, but I hate silent bugs. Given the existing pattern of "external preallocation", it seems like a fragile interface worth asserting our expectations. The panic_on_warn folks will get exactly what they wanted: immediate feedback on "expected to be impossible" cases: https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on > > > > + 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. > > There are a few sysfs files that userspace can read from out there :) Yes, but the vmap area is also used by default for process stacks, etc. Malicious fragmentation is already possible. I understood the concern to be about "regular" use. (And if I'm wrong, we can add a knob maybe?) > > > 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). > > Ah, "magic", gotta love it... Yeeeah. :P -- Kees Cook
