From: Michal Hocko <mhocko@suse.com>
To: Kees Cook <keescook@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Alexey Dobriyan <adobriyan@gmail.com>,
Lee Duncan <lduncan@suse.com>, Chris Leech <cleech@redhat.com>,
Adam Nichols <adam@grimm-co.com>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2] seq_file: Unconditionally use vmalloc for buffer
Date: Tue, 16 Mar 2021 09:31:23 +0100 [thread overview]
Message-ID: <YFBs202BqG9uqify@dhcp22.suse.cz> (raw)
In-Reply-To: <20210315174851.622228-1-keescook@chromium.org>
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@chromium.org>
> ---
> 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
next prev parent reply other threads:[~2021-03-16 8:32 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 17:48 [PATCH v2] seq_file: Unconditionally use vmalloc for buffer Kees Cook
2021-03-15 18:33 ` Al Viro
2021-03-15 20:43 ` Kees Cook
2021-03-16 7:24 ` Greg Kroah-Hartman
2021-03-16 12:43 ` Al Viro
2021-03-16 12:55 ` Greg Kroah-Hartman
2021-03-16 13:01 ` Michal Hocko
2021-03-16 19:18 ` Kees Cook
2021-03-17 10:44 ` Greg Kroah-Hartman
2021-03-16 8:31 ` Michal Hocko [this message]
2021-03-16 19:08 ` Kees Cook
2021-03-17 12:08 ` Michal Hocko
2021-03-17 13:34 ` Greg Kroah-Hartman
2021-03-17 14:44 ` Michal Hocko
2021-03-17 14:56 ` Greg Kroah-Hartman
2021-03-17 15:20 ` Michal Hocko
2021-03-17 15:38 ` Greg Kroah-Hartman
2021-03-17 15:48 ` Michal Hocko
2021-03-17 21:30 ` Kees Cook
2021-03-18 8:07 ` Greg Kroah-Hartman
2021-03-18 15:51 ` Kees Cook
2021-03-18 17:56 ` Greg Kroah-Hartman
2021-03-19 14:07 ` [seq_file] 5fd6060e50: stress-ng.eventfd.ops_per_sec -49.1% regression kernel test robot
2021-03-19 19:31 ` Kees Cook
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