Re: [syzbot] KASAN: null-ptr-deref Read in __free_pages

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

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On domenica 20 marzo 2022 23:04:18 CET syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    56e337f2cf13 Revert "gpio: Revert regression in sysfs-gpio..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17e0a8c5700000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d35f9bc6884af6c9
> dashboard link: https://syzkaller.appspot.com/bug?extid=d55757faa9b80590767b
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=134e4e91700000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=127bd083700000
> 
> Bisection is inconclusive: the issue happens on the oldest tested release.
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=138601c5700000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=104601c5700000
> console output: https://syzkaller.appspot.com/x/log.txt?x=178601c5700000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+d55757faa9b80590767b@syzkaller.appspotmail.com
> 
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007ffc794ecea0 R14: 000000000000000d R15: 00007ff328525490
>  </TASK>
> ==================================================================
> BUG: KASAN: null-ptr-deref in instrument_atomic_read include/linux/instrumented.h:71 [inline]
> BUG: KASAN: null-ptr-deref in atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
> BUG: KASAN: null-ptr-deref in page_ref_count include/linux/page_ref.h:67 [inline]
> BUG: KASAN: null-ptr-deref in put_page_testzero include/linux/mm.h:717 [inline]
> BUG: KASAN: null-ptr-deref in __free_pages+0x1f/0x1b0 mm/page_alloc.c:5473
> Read of size 4 at addr 0000000000000034 by task syz-executor168/3599
> 
> CPU: 1 PID: 3599 Comm: syz-executor168 Not tainted 5.17.0-rc8-syzkaller-00003-g56e337f2cf13 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  <TASK>
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
>  __kasan_report mm/kasan/report.c:446 [inline]
>  kasan_report.cold+0x66/0xdf mm/kasan/report.c:459
>  check_region_inline mm/kasan/generic.c:183 [inline]
>  kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
>  instrument_atomic_read include/linux/instrumented.h:71 [inline]
>  atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
>  page_ref_count include/linux/page_ref.h:67 [inline]
>  put_page_testzero include/linux/mm.h:717 [inline]
>  __free_pages+0x1f/0x1b0 mm/page_alloc.c:5473
>  watch_queue_set_size+0x499/0x630 kernel/watch_queue.c:275
>  pipe_ioctl+0xac/0x2b0 fs/pipe.c:632
>  vfs_ioctl fs/ioctl.c:51 [inline]
>  __do_sys_ioctl fs/ioctl.c:874 [inline]
>  __se_sys_ioctl fs/ioctl.c:860 [inline]
>  __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:860
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7ff3284b2d59
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 31 18 00 00 90 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 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007ffc794ece38 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000000000031 RCX: 00007ff3284b2d59
> RDX: 0000000000000029 RSI: 0000000000005760 RDI: 0000000000000003
> RBP: 00007ffc794ece40 R08: 0000000000000001 R09: 00007ffc794e0035
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> R13: 00007ffc794ecea0 R14: 000000000000000d R15: 00007ff328525490
>  </TASK>
> ==================================================================
> 
> 
> ---
> This report 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 issue. See:
> https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches
> 
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> 
Probably I'm missing something, but I want to try once more...

The argument I follow is that when, in the "error_p" label, the code 
frees the pages, it frees in a loop from pages[0] to pages[nr_pages -1].
Thus I saw that when pages[i] are allocated at line 256 by a call to 
alloc_page() and this allocation fails it jumps to the "error_p" label
without saving and using the number of successful allocations as a limit
of the loop that calls __free_pages().

As I said, perhaps I'm missing something deeper but I noticed that in my
diff I assigned "allocated_pages" with "i + 1" and probably I'm still 
going off by one.

Let's try the last time...

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

Thanks,

Fabio M. De Francesco

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diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 00703444a219..5b0531020cf8 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -220,7 +220,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
 	struct page **pages;
 	unsigned long *bitmap;
 	unsigned long user_bufs;
-	int ret, i, nr_pages;
+	int ret, i, nr_pages, allocated_pages;
 
 	if (!wqueue)
 		return -ENODEV;
@@ -254,6 +254,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
 
 	for (i = 0; i < nr_pages; i++) {
 		pages[i] = alloc_page(GFP_KERNEL);
+		allocated_pages = i;
 		if (!pages[i])
 			goto error_p;
 		pages[i]->index = i * WATCH_QUEUE_NOTES_PER_PAGE;
@@ -271,7 +272,7 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)
 	return 0;
 
 error_p:
-	for (i = 0; i < nr_pages; i++)
+	for (i = 0; i < allocated_pages; i++)
 		__free_page(pages[i]);
 	kfree(pages);
 error: