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SubjectRe: [PATCH] libfs: fix infoleak in simple_attr_read()
On Fri, Mar 13, 2020 at 09:45:11AM -0700, Eric Biggers wrote:
> On Sat, Mar 07, 2020 at 06:38:49PM -0800, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> >
> > Reading from a debugfs file at a nonzero position, without first reading
> > at position 0, leaks uninitialized memory to userspace.
> >
> > It's a bit tricky to do this, since lseek() and pread() aren't allowed
> > on these files, and write() doesn't update the position on them. But
> > writing to them with splice() *does* update the position:
> >
> > #define _GNU_SOURCE 1
> > #include <fcntl.h>
> > #include <stdio.h>
> > #include <unistd.h>
> > int main()
> > {
> > int pipes[2], fd, n, i;
> > char buf[32];
> >
> > pipe(pipes);
> > write(pipes[1], "0", 1);
> > fd = open("/sys/kernel/debug/fault_around_bytes", O_RDWR);
> > splice(pipes[0], NULL, fd, NULL, 1, 0);
> > n = read(fd, buf, sizeof(buf));
> > for (i = 0; i < n; i++)
> > printf("%02x", buf[i]);
> > printf("\n");
> > }
> >
> > Output:
> > 5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a5a30
> >
> > Fix the infoleak by making simple_attr_read() always fill
> > simple_attr::get_buf if it hasn't been filled yet.
> >
> > Reported-by: syzbot+fcab69d1ada3e8d6f06b@syzkaller.appspotmail.com
> > Reported-by: Alexander Potapenko <glider@google.com>
> > Fixes: acaefc25d21f ("[PATCH] libfs: add simple attribute files")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > ---
> > fs/libfs.c | 8 +++++---
> > 1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/fs/libfs.c b/fs/libfs.c
> > index c686bd9caac6..3759fbacf522 100644
> > --- a/fs/libfs.c
> > +++ b/fs/libfs.c
> > @@ -891,7 +891,7 @@ int simple_attr_open(struct inode *inode, struct file *file,
> > {
> > struct simple_attr *attr;
> >
> > - attr = kmalloc(sizeof(*attr), GFP_KERNEL);
> > + attr = kzalloc(sizeof(*attr), GFP_KERNEL);
> > if (!attr)
> > return -ENOMEM;
> >
> > @@ -931,9 +931,11 @@ ssize_t simple_attr_read(struct file *file, char __user *buf,
> > if (ret)
> > return ret;
> >
> > - if (*ppos) { /* continued read */
> > + if (*ppos && attr->get_buf[0]) {
> > + /* continued read */
> > size = strlen(attr->get_buf);
> > - } else { /* first read */
> > + } else {
> > + /* first read */
> > u64 val;
> > ret = attr->get(attr->data, &val);
> > if (ret)
> > --
> > 2.25.1
>
> Any comments on this? Al, seems this is something you should pick up?
>
> - Eric

Ping.

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