On Mon 15-03-21 10:48:51, Kees Cook wrote: > The sysfs interface to seq_file continues to be rather fragile, 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 > seq_file already uses kvmalloc(), is almost always using a PAGE_SIZE or > larger allocation, has allocations are normally short lived, and is not > normally on a performance critical path. I have already objected without having my concerns really addressed. Your observation that most of buffers are PAGE_SIZE in the vast majority cases matches my experience and kmalloc should perform better than vmalloc. You should check the most common /proc readers at least. Also this cannot really be done for configurations with a very limited vmalloc space (32b for example). Those systems are more and more rare but you shouldn't really allow userspace to deplete the vmalloc space. I would be also curious to see how vmalloc scales with huge number of single page allocations which would be easy to trigger with this patch. > [1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > --- > fs/seq_file.c | 15 ++++++++++----- > 1 file changed, 10 insertions(+), 5 deletions(-) > > diff --git a/fs/seq_file.c b/fs/seq_file.c > index cb11a34fb871..16fb4a4e61e3 100644 > --- a/fs/seq_file.c > +++ b/fs/seq_file.c > @@ -32,7 +32,12 @@ static void seq_set_overflow(struct seq_file *m) > > static void *seq_buf_alloc(unsigned long size) > { > - return kvmalloc(size, GFP_KERNEL_ACCOUNT); > + /* > + * To be proactively defensive against buggy seq_get_buf() callers > + * (i.e. sysfs handlers), use the vmap area to gain the trailing > + * guard page which will protect against linear buffer overflows. > + */ > + return __vmalloc(size, GFP_KERNEL_ACCOUNT); > } > > /** > @@ -130,7 +135,7 @@ static int traverse(struct seq_file *m, loff_t offset) > > Eoverflow: > m->op->stop(m, p); > - kvfree(m->buf); > + vfree(m->buf); > m->count = 0; > m->buf = seq_buf_alloc(m->size <<= 1); > return !m->buf ? -ENOMEM : -EAGAIN; > @@ -237,7 +242,7 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter) > goto Fill; > // need a bigger buffer > m->op->stop(m, p); > - kvfree(m->buf); > + vfree(m->buf); > m->count = 0; > m->buf = seq_buf_alloc(m->size <<= 1); > if (!m->buf) > @@ -349,7 +354,7 @@ EXPORT_SYMBOL(seq_lseek); > int seq_release(struct inode *inode, struct file *file) > { > struct seq_file *m = file->private_data; > - kvfree(m->buf); > + vfree(m->buf); > kmem_cache_free(seq_file_cache, m); > return 0; > } > @@ -585,7 +590,7 @@ int single_open_size(struct file *file, int (*show)(struct seq_file *, void *), > return -ENOMEM; > ret = single_open(file, show, data); > if (ret) { > - kvfree(buf); > + vfree(buf); > return ret; > } > ((struct seq_file *)file->private_data)->buf = buf; > -- > 2.25.1 -- Michal Hocko SUSE Labs
