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Date: Mon, 11 Oct 2021 22:08:14 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Al Viro <viro@...iv.linux.org.uk>,
Andreas Gruenbacher <agruenba@...hat.com>,
Christoph Hellwig <hch@...radead.org>,
"Darrick J. Wong" <djwong@...nel.org>, Jan Kara <jack@...e.cz>,
Matthew Wilcox <willy@...radead.org>,
cluster-devel <cluster-devel@...hat.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
Josef Bacik <josef@...icpanda.com>,
Will Deacon <will@...nel.org>
Subject: Re: [RFC][arm64] possible infinite loop in btrfs search_ioctl()
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@....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
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