On Mon, Mar 15, 2021 at 09:34:18AM +0100, Michal Hocko wrote: > On Fri 12-03-21 12:55:58, 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(), that allocations are normally short > > lived, and that they are not normally performance critical. > > What is the runtime effect of this change? The interface is widely used
I haven't been able to measure any differences yet, but maybe I lack imagination about workloads that are heavy on /sys or /proc accesses. > for many other interfaces - e.g. in proc. While from the correctness POV > this should be OK (ish for 64b it is definitely problem for kernels with > lowmem and limited vmalloc space). Vmalloc is also to be expected to > regress in performance for small allocations which is the most usual > case. seq_file's default size is PAGE_SIZE (and just goes up by powers of 2 from there), with the rare (3 callers) exception of single_open_size(), which for at least 1 case is always >PAGE_SIZE. (I realize PAGE_SIZE may be considered "small" for vmalloc, but I think gaining the guard page is worth it, given the recurring flaws we see with at least sysfs handlers.) -Kees > > > [1] https://blog.grimm-co.com/2021/03/new-old-bugs-in-linux-kernel.html > > > > Signed-off-by: Kees Cook <keesc...@chromium.org> > > --- > > fs/seq_file.c | 10 +++++----- > > 1 file changed, 5 insertions(+), 5 deletions(-) > > > > diff --git a/fs/seq_file.c b/fs/seq_file.c > > index cb11a34fb871..ad78577d4c2c 100644 > > --- a/fs/seq_file.c > > +++ b/fs/seq_file.c > > @@ -32,7 +32,7 @@ static void seq_set_overflow(struct seq_file *m) > > > > static void *seq_buf_alloc(unsigned long size) > > { > > - return kvmalloc(size, GFP_KERNEL_ACCOUNT); > > + return __vmalloc(size, GFP_KERNEL_ACCOUNT); > > } > > > > /** > > @@ -130,7 +130,7 @@ 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 +237,7 @@ 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 +349,7 @@ 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 +585,7 @@ 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 -- Kees Cook
