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Date:   Mon, 11 Oct 2021 22:08:14 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Darrick J. Wong" <djwong@kernel.org>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <willy@infradead.org>,
        cluster-devel <cluster-devel@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "ocfs2-devel@oss.oracle.com" <ocfs2-devel@oss.oracle.com>,
        Josef Bacik <josef@toxicpanda.com>,
        Will Deacon <will@kernel.org>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()
Message-ID: <YWSnvq58jDsDuIik@arm.com>
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On Mon, Oct 11, 2021 at 12:15:43PM -0700, Linus Torvalds wrote:
> On Mon, Oct 11, 2021 at 10:38 AM Catalin Marinas
> <catalin.marinas@arm.com> wrote:
> > I cleaned up this patch [1] but I realised it still doesn't solve it.
> > The arm64 __copy_to_user_inatomic(), while ensuring progress if called
> > in a loop, it does not guarantee precise copy to the fault position.
> 
> That should be ok., We've always allowed the user copy to return early
> if it does word copies and hits a page crosser that causes a fault.
> 
> Any user then has the choice of:
> 
>  - partial copies are bad
> 
>  - partial copies are handled and then you retry from the place
> copy_to_user() failed at
> 
> and in that second case, the next time around, you'll get the fault
> immediately (or you'll make some more progress - maybe the user copy
> loop did something different just because the length and/or alignment
> was different).
> 
> If you get the fault immediately, that's -EFAULT.
> 
> And if you make some more progress, it's again up to the caller to
> rinse and repeat.
> 
> End result: user copy functions do not have to report errors exactly.
> It is the caller that has to handle the situation.
> 
> Most importantly: "exact error or not" doesn't actually _matter_ to
> the caller. If the caller does the right thing for an exact error, it
> will do the right thing for an inexact one too. See above.

Yes, that's my expectation (though fixed fairly recently in the arm64
user copy routines).

> > The copy_to_sk(), after returning an error, starts again from the previous
> > sizeof(sh) boundary rather than from where the __copy_to_user_inatomic()
> > stopped. So it can get stuck attempting to copy the same search header.
> 
> That seems to be purely a copy_to_sk() bug.
> 
> Or rather, it looks like a bug in the caller. copy_to_sk() itself does
> 
>                 if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) {
>                         ret = 0;
>                         goto out;
>                 }
> 
> and the comment says
> 
>          *  0: all items from this leaf copied, continue with next
> 
> but that comment is then obviously not actually true in that it's not
> "continue with next" at all.

The comments were correct before commit a48b73eca4ce ("btrfs: fix
potential deadlock in the search ioctl") which introduced the
potentially infinite loop.

Something like this would make the comments match (I think):

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index cc61813213d8..1debf6a124e8 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2161,7 +2161,7 @@ static noinline int copy_to_sk(struct btrfs_path *path,
 		 * properly this next time through
 		 */
 		if (copy_to_user_nofault(ubuf + *sk_offset, &sh, sizeof(sh))) {
-			ret = 0;
+			ret = -EFAULT;
 			goto out;
 		}

@@ -2175,7 +2175,7 @@ static noinline int copy_to_sk(struct btrfs_path *path,
 			 */
 			if (read_extent_buffer_to_user_nofault(leaf, up,
 						item_off, item_len)) {
-				ret = 0;
+				ret = -EFAULT;
 				*sk_offset -= sizeof(sh);
 				goto out;
 			}
@@ -2260,12 +2260,8 @@ static noinline int search_ioctl(struct inode *inode,
 	key.type = sk->min_type;
 	key.offset = sk->min_offset;

-	while (1) {
-		ret = fault_in_pages_writeable(ubuf + sk_offset,
-					       *buf_size - sk_offset);
-		if (ret)
-			break;
-
+	ret = fault_in_pages_writeable(ubuf, *buf_size);
+	while (ret == 0) {
 		ret = btrfs_search_forward(root, &key, path, sk->min_transid);
 		if (ret != 0) {
 			if (ret > 0)
@@ -2275,9 +2271,14 @@ static noinline int search_ioctl(struct inode *inode,
 		ret = copy_to_sk(path, &key, sk, buf_size, ubuf,
 				 &sk_offset, &num_found);
 		btrfs_release_path(path);
-		if (ret)
-			break;

+		/*
+		 * Fault in copy_to_sk(), attempt to bring the page in after
+		 * releasing the locks and retry.
+		 */
+		if (ret == -EFAULT)
+			ret = fault_in_pages_writeable(ubuf + sk_offset,
+				       sizeof(struct btrfs_ioctl_search_header));
 	}
 	if (ret > 0)
 		ret = 0;

> > An ugly fix is to fall back to byte by byte copying so that we can
> > attempt the actual fault address in fault_in_pages_writeable().
> 
> No, changing the user copy machinery is wrong. The caller really has
> to do the right thing with partial results.
> 
> And I think we need to make fault_in_pages_writeable() match the
> actual faulting cases - maybe remote the "pages" part of the name?

Ah, good point. Without removing "pages" from the name (too late over
here to grep the kernel), something like below:

diff --git a/arch/arm64/include/asm/page-def.h b/arch/arm64/include/asm/page-def.h
index 2403f7b4cdbf..3768ac4a6610 100644
--- a/arch/arm64/include/asm/page-def.h
+++ b/arch/arm64/include/asm/page-def.h
@@ -15,4 +15,9 @@
 #define PAGE_SIZE		(_AC(1, UL) << PAGE_SHIFT)
 #define PAGE_MASK		(~(PAGE_SIZE-1))
 
+#ifdef CONFIG_ARM64_MTE
+#define FAULT_GRANULE_SIZE	(16)
+#define FAULT_GRANULE_MASK	(~(FAULT_GRANULE_SIZE-1))
+#endif
+
 #endif /* __ASM_PAGE_DEF_H */
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 62db6b0176b9..7aef732e4fa7 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -16,6 +16,11 @@
 #include <linux/hardirq.h> /* for in_interrupt() */
 #include <linux/hugetlb_inline.h>
 
+#ifndef FAULT_GRANULE_SIZE
+#define FAULT_GRANULE_SIZE	PAGE_SIZE
+#define FAULT_GRANULE_MASK	PAGE_MASK
+#endif
+
 struct pagevec;
 
 static inline bool mapping_empty(struct address_space *mapping)
@@ -751,12 +756,12 @@ static inline int fault_in_pages_writeable(char __user *uaddr, size_t size)
 	do {
 		if (unlikely(__put_user(0, uaddr) != 0))
 			return -EFAULT;
-		uaddr += PAGE_SIZE;
+		uaddr += FAULT_GRANULE_SIZE;
 	} while (uaddr <= end);
 
-	/* Check whether the range spilled into the next page. */
-	if (((unsigned long)uaddr & PAGE_MASK) ==
-			((unsigned long)end & PAGE_MASK))
+	/* Check whether the range spilled into the next granule. */
+	if (((unsigned long)uaddr & FAULT_GRANULE_MASK) ==
+			((unsigned long)end & FAULT_GRANULE_MASK))
 		return __put_user(0, end);
 
 	return 0;

If this looks in the right direction, I'll do some proper patches
tomorrow.

-- 
Catalin