On Wed 31-03-21 19:21:45, 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.
I thought the previous discussion has led to a conclusion that the preferred way is to disallow direct seq_file buffer usage. But this is obviously up to sysfs maintainers. I am happy you do not want to spread this out to all seq_file users anymore. > 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. Let me clarify on this, because this is not quite right. kvmalloc vs vmalloc (both with GFP_KERNEL) on PAGE_SIZE are two different beasts. The first one is almost always going to use kmalloc because the page allocator almost never fails those requests. > 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 <[email protected]> > --- > v3: > - Limit to only sysfs (instead of all of seq_file). > v2: > https://lore.kernel.org/lkml/[email protected]/ > v1: > https://lore.kernel.org/lkml/[email protected]/ > --- > 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); > + sf->buf = __vmalloc(kn->attr.size, GFP_KERNEL_ACCOUNT); > + if (!sf->buf) > + return ERR_PTR(-ENOMEM); > + sf->size = kn->attr.size; > + > + return NULL + !*ppos; > +} > + > /* > * Reads on sysfs are handled through seq_file, which takes care of hairy > * details like buffering and seeking. The following function pipes > @@ -206,14 +226,17 @@ static const struct kernfs_ops sysfs_file_kfops_empty = > { > }; > > static const struct kernfs_ops sysfs_file_kfops_ro = { > + .seq_start = sysfs_kf_seq_start, > .seq_show = sysfs_kf_seq_show, > }; > > static const struct kernfs_ops sysfs_file_kfops_wo = { > + .seq_start = sysfs_kf_seq_start, > .write = sysfs_kf_write, > }; > > static const struct kernfs_ops sysfs_file_kfops_rw = { > + .seq_start = sysfs_kf_seq_start, > .seq_show = sysfs_kf_seq_show, > .write = sysfs_kf_write, > }; > -- > 2.25.1 -- Michal Hocko SUSE Labs
