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From: Catalin Marinas <catalin.marinas@arm.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: cluster-devel <cluster-devel@redhat.com>, Jan Kara <jack@suse.cz>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Will Deacon <will@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Josef Bacik <josef@toxicpanda.com>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>
Subject: Re: [Ocfs2-devel] [RFC][arm64] possible infinite loop in btrfs search_ioctl()
Date: Tue, 31 Aug 2021 14:54:50 +0100	[thread overview]
Message-ID: <YS40qqmXL7CMFLGq@arm.com> (raw)
In-Reply-To: <YSqOUb7yZ7kBoKRY@zeniv-ca.linux.org.uk>

On Sat, Aug 28, 2021 at 08:28:17PM +0100, Al Viro wrote:
> 	AFAICS, a48b73eca4ce "btrfs: fix potential deadlock in the search ioctl"
> has introduced a bug at least on arm64.
> 
> Relevant bits: in search_ioctl() we have
>         while (1) {
>                 ret = fault_in_pages_writeable(ubuf + sk_offset,
>                                                *buf_size - sk_offset);
>                 if (ret)
>                         break;
> 
>                 ret = btrfs_search_forward(root, &key, path, sk->min_transid);
>                 if (ret != 0) {
>                         if (ret > 0)
>                                 ret = 0;
>                         goto err;
>                 }
>                 ret = copy_to_sk(path, &key, sk, buf_size, ubuf,
>                                  &sk_offset, &num_found);
>                 btrfs_release_path(path);
>                 if (ret)
>                         break;
> 
>         }
> and in copy_to_sk() -
>                 sh.objectid = key->objectid;
>                 sh.offset = key->offset;
>                 sh.type = key->type;
>                 sh.len = item_len;
>                 sh.transid = found_transid;
> 
>                 /*
>                  * Copy search result header. If we fault then loop again so we
>                  * can fault in the pages and -EFAULT there if there's a
>                  * problem. Otherwise we'll fault and then copy the buffer in
>                  * properly this next time through
>                  */
>                 if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) {
>                         ret = 0;
>                         goto out;
>                 }
> with sk_offset left unchanged if the very first copy_to_user_nofault() fails.
> 
> Now, consider a situation on arm64 where ubuf points to the beginning of page,
> ubuf[0] can be accessed, but ubuf[16] can not (possible with MTE, AFAICS).  We do
> fault_in_pages_writeable(), which succeeds.  When we get to copy_to_user_nofault()
> we fail as soon as it gets past the first 16 bytes.  And we repeat everything from
> scratch, with no progress made, since short copies are treated as "discard and
> repeat" here.

So if copy_to_user_nofault() returns -EFAULT, copy_to_sk() returns 0
(following commit a48b73eca4ce). I think you are right, search_ioctl()
can get into an infinite loop attempting to write to user if the
architecture can trigger faults at smaller granularity than the page
boundary. fault_in_pages_writeable() won't fix it if ubuf[0] is
writable and doesn't trigger an MTE tag check fault.

An arm64-specific workaround would be for pagefault_disable() to disable
tag checking. It's a pretty big hammer, weakening the out of bounds
access detection of MTE. My preference would be a fix in the btrfs code.

A btrfs option would be for copy_to_sk() to return an indication of
where the fault occurred and get fault_in_pages_writeable() to check
that location, even if the copying would restart from an earlier offset
(this requires open-coding copy_to_user_nofault()). An attempt below,
untested and does not cover read_extent_buffer_to_user_nofault():

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 0ba98e08a029..9e74ba1c955d 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2079,6 +2079,7 @@ static noinline int copy_to_sk(struct btrfs_path *path,
 			       size_t *buf_size,
 			       char __user *ubuf,
 			       unsigned long *sk_offset,
+			       unsigned long *fault_offset,
 			       int *num_found)
 {
 	u64 found_transid;
@@ -2143,7 +2144,11 @@ static noinline int copy_to_sk(struct btrfs_path *path,
 		 * problem. Otherwise we'll fault and then copy the buffer in
 		 * properly this next time through
 		 */
-		if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) {
+		pagefault_disable();
+		ret = __copy_to_user_inatomic(ubuf + *sk_offset, &sh, sizeof(sh));
+		pagefault_enable();
+		*fault_offset = *sk_offset + sizeof(sh) - ret;
+		if (ret) {
 			ret = 0;
 			goto out;
 		}
@@ -2218,6 +2223,7 @@ static noinline int search_ioctl(struct inode *inode,
 	int ret;
 	int num_found = 0;
 	unsigned long sk_offset = 0;
+	unsigned long fault_offset = 0;

 	if (*buf_size < sizeof(struct btrfs_ioctl_search_header)) {
 		*buf_size = sizeof(struct btrfs_ioctl_search_header);
@@ -2244,8 +2250,8 @@ static noinline int search_ioctl(struct inode *inode,
 	key.offset = sk->min_offset;

 	while (1) {
-		ret = fault_in_pages_writeable(ubuf + sk_offset,
-					       *buf_size - sk_offset);
+		ret = fault_in_pages_writeable(ubuf + fault_offset,
+					       *buf_size - fault_offset);
 		if (ret)
 			break;

@@ -2256,7 +2262,7 @@ static noinline int search_ioctl(struct inode *inode,
 			goto err;
 		}
 		ret = copy_to_sk(path, &key, sk, buf_size, ubuf,
-				 &sk_offset, &num_found);
+				 &sk_offset, &fault_offset, &num_found);
 		btrfs_release_path(path);
 		if (ret)
 			break;

-- 
Catalin

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  reply	other threads:[~2021-09-13 14:29 UTC|newest]

Thread overview: 100+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-27 16:49 [Ocfs2-devel] [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 01/19] iov_iter: Fix iov_iter_get_pages{, _alloc} page fault return value Andreas Gruenbacher
2021-09-09 11:09   ` Christoph Hellwig
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 02/19] powerpc/kvm: Fix kvm_use_magic_page Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 03/19] gup: Turn fault_in_pages_{readable, writeable} into fault_in_{readable, writeable} Andreas Gruenbacher
2021-08-27 19:08   ` Al Viro
2021-09-03 14:56   ` Filipe Manana
2021-09-28 15:02     ` Andreas Gruenbacher
2021-09-28 16:37       ` Matthew Wilcox
2021-09-28 20:41         ` Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 04/19] iov_iter: Turn iov_iter_fault_in_readable into fault_in_iov_iter_readable Andreas Gruenbacher
2021-08-27 18:53   ` Al Viro
2021-08-27 18:57     ` Linus Torvalds
2021-08-27 19:16       ` Al Viro
2021-08-27 20:56   ` Kari Argillander
2021-08-28 17:13     ` Linus Torvalds
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Andreas Gruenbacher
2021-08-27 18:49   ` Al Viro
2021-08-27 19:05     ` Linus Torvalds
2021-08-27 19:23       ` Al Viro
2021-08-27 19:33         ` Linus Torvalds
2021-08-27 19:37           ` Al Viro
2021-08-27 21:48             ` Al Viro
2021-08-27 21:57               ` Al Viro
2021-08-27 23:22                 ` Luck, Tony
2021-08-28  2:20                   ` Luck, Tony
2021-08-28 21:47                   ` Thomas Gleixner
2021-08-28 22:04                     ` Al Viro
2021-08-28 22:11                       ` Al Viro
2021-08-28 22:19                         ` Al Viro
2021-08-28 22:51                           ` Al Viro
2021-08-29 18:44                             ` Thomas Gleixner
2021-08-29 19:46                               ` Al Viro
2021-08-29 19:51                                 ` Thomas Gleixner
2021-08-28 22:20                         ` Tony Luck
2021-08-29  1:40                           ` Matthew Wilcox
2021-08-30 15:41                             ` Luck, Tony
2021-08-28 22:23                       ` Thomas Gleixner
2021-08-28 19:28               ` [Ocfs2-devel] [RFC][arm64] possible infinite loop in btrfs search_ioctl() Al Viro
2021-08-31 13:54                 ` Catalin Marinas [this message]
2021-08-31 15:28                   ` Al Viro
2021-08-31 16:01                     ` Catalin Marinas
2021-10-11 17:37                     ` Catalin Marinas
2021-10-11 19:15                       ` Linus Torvalds
2021-10-11 21:08                         ` Catalin Marinas
2021-10-11 23:59                           ` Linus Torvalds
2021-10-12 17:27                             ` Catalin Marinas
2021-10-12 17:58                               ` Linus Torvalds
2021-10-18 17:13                                 ` Catalin Marinas
2021-10-21  0:46                             ` Andreas Gruenbacher
2021-10-21 10:05                               ` Catalin Marinas
2021-10-21 14:42                                 ` Andreas Gruenbacher
2021-10-21 17:09                                   ` Catalin Marinas
2021-10-21 18:00                                     ` Andreas Gruenbacher
2021-10-22 18:41                                       ` Catalin Marinas
2021-10-25 19:37                                         ` Andreas Gruenbacher
2021-10-22  2:30                                   ` Linus Torvalds
2021-10-22  9:34                                     ` Catalin Marinas
2021-08-29  0:58               ` [Ocfs2-devel] [PATCH v7 05/19] iov_iter: Introduce fault_in_iov_iter_writeable Al Viro
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 06/19] gfs2: Add wrapper for iomap_file_buffered_write Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 07/19] gfs2: Clean up function may_grant Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 08/19] gfs2: Eliminate vestigial HIF_FIRST Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 09/19] gfs2: Remove redundant check from gfs2_glock_dq Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 10/19] gfs2: Introduce flag for glock holder auto-demotion Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 11/19] gfs2: Move the inode glock locking to gfs2_file_buffered_write Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 12/19] gfs2: Eliminate ip->i_gh Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 13/19] gfs2: Fix mmap + page fault deadlocks for buffered I/O Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 14/19] iomap: Fix iomap_dio_rw return value for user copies Andreas Gruenbacher
2021-09-03 18:54   ` Darrick J. Wong
2021-09-09 11:17   ` Christoph Hellwig
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 15/19] iomap: Support partial direct I/O on user copy failures Andreas Gruenbacher
2021-09-03 18:54   ` Darrick J. Wong
2021-09-09 11:20   ` Christoph Hellwig
2021-09-28 15:05     ` Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 16/19] iomap: Add done_before argument to iomap_dio_rw Andreas Gruenbacher
2021-08-27 18:30   ` Darrick J. Wong
2021-08-27 20:15     ` Andreas Gruenbacher
2021-08-27 21:32       ` Darrick J. Wong
2021-08-27 21:49         ` Andreas Grünbacher
2021-08-27 22:35         ` Linus Torvalds
2021-09-03 18:47           ` Darrick J. Wong
2021-09-03 18:53   ` Darrick J. Wong
2021-09-09 11:30   ` Christoph Hellwig
2021-09-09 17:22     ` Linus Torvalds
2021-09-10  7:36       ` Christoph Hellwig
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 17/19] gup: Introduce FOLL_NOFAULT flag to disable page faults Andreas Gruenbacher
2021-09-09 11:36   ` Christoph Hellwig
2021-09-09 17:17     ` Linus Torvalds
2021-09-10  7:24       ` Christoph Hellwig
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 18/19] iov_iter: Introduce nofault " Andreas Gruenbacher
2021-08-27 18:47   ` Al Viro
2021-08-27 19:56     ` Andreas Gruenbacher
2021-08-27 16:49 ` [Ocfs2-devel] [PATCH v7 19/19] gfs2: Fix mmap + page fault deadlocks for direct I/O Andreas Gruenbacher
2021-08-27 17:16 ` [Ocfs2-devel] [PATCH v7 00/19] gfs2: Fix mmap + page fault deadlocks Linus Torvalds
2021-09-01 19:52   ` Andreas Gruenbacher
2021-09-03 15:52     ` Linus Torvalds
2021-09-03 18:25       ` Al Viro
2021-09-03 18:47         ` Linus Torvalds
2021-09-03 19:51       ` Andreas Grünbacher
2021-09-03 15:07 ` Filipe Manana

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