linux-kernel - Re: [syzbot] KASAN: null-ptr-deref Read in __free

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Date:   Mon, 21 Mar 2022 08:46:51 +0000
From:   David Howells <dhowells@...hat.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     dhowells@...hat.com, christophe.jaillet@...adoo.fr,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] KASAN: null-ptr-deref Read in __free_pages

Hi Fabio,

How about using my patch instead?  I wonder if __free_pages() should check for
a NULL pages pointer...

David
---
watch_queue: Fix NULL dereference in error cleanup

In watch_queue_set_size(), the error cleanup code doesn't take account of
the fact that __free_page() can't handle a NULL pointer when trying to free
up buffer pages that did get allocated.

Fix this by only calling __free_page() on non-NULL pointers extracted from
the list (the list was allocated by kcalloc() so was pre-cleared).

Without the fix, this can lead to something like the following:

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
...
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

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-and-tested-by: syzbot+d55757faa9b80590767b@...kaller.appspotmail.com
Signed-off-by: David Howells <dhowells@...hat.com>
---
 kernel/watch_queue.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index 00703444a219..09f35bab496e 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -272,7 +272,8 @@ long watch_queue_set_size(struct pipe_inode_info *pipe, unsigned int nr_notes)

 error_p:
 	for (i = 0; i < nr_pages; i++)
-		__free_page(pages[i]);
+		if (pages[i])
+			__free_page(pages[i]);
 	kfree(pages);
 error:
 	(void) account_pipe_buffers(pipe->user, nr_pages, pipe->nr_accounted);