On Sat, Mar 07, 2020 at 06:38:49PM -0800, Eric Biggers wrote: > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx > Reported-by: Alexander Potapenko <glider@xxxxxxxxxx> > Fixes: acaefc25d21f ("[PATCH] libfs: add simple attribute files") > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> Yikes, that's an important fix! Acked-by: Kees Cook <keescook@xxxxxxxxxxxx> Luckily (as Alexander mentioned too), most distros make debugfs non-accessible by non-root (I hope): $ ls -lda /sys/kernel/debug drwx------ 39 root root 0 Jan 8 09:10 /sys/kernel/debug/ That function is also exposed via DEFINE_SIMPLE_ATTRIBUTE(), but those users appear to also be mostly (all?) debugfs too. And, just to note, for v5.3 and later, this would be fully mitigated by booting with "init_on_alloc=1". -Kees > --- > 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 > -- Kees Cook