On Thu, Jun 24, 2021 at 12:15:46PM +0100, Matthew Wilcox wrote:
> On Thu, Jun 24, 2021 at 08:04:07AM +0100, Christoph Hellwig wrote:
> > On Thu, Jun 24, 2021 at 04:24:46AM +0100, Matthew Wilcox wrote:
> > > On Thu, Jun 24, 2021 at 11:10:41AM +0800, Chen Huang wrote:
> > > > In userspace, I perform such operation:
> > > >
> > > > fd = open("/tmp/test", O_RDWR | O_SYNC);
> > > > access_address = (char *)mmap(NULL, uio_size, PROT_READ, MAP_SHARED, uio_fd, 0);
> > > > ret = write(fd, access_address + 2, sizeof(long));
> > >
> > > ... you know that accessing this at unaligned offsets isn't going to
> > > work. It's completely meaningless. Why are you trying to do it?
> >
> > We still should not cause an infinite loop in kernel space due to a
> > a userspace programmer error.
>
> They're running as root and they've mapped some device memory. We can't
> save them from themself. Imagine if they'd done this to the NVMe BAR.
Ignoring the MMIO case for now, I can trigger the same infinite loop
with MTE (memory tagging), something like:
char *a;
a = mmap(0, page_sz, PROT_READ | PROT_WRITE | PROT_MTE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
/* tag 0 is the default, set tag 1 for the next 16 bytes */
set_tag((unsigned long)(a + 16) | (1UL << 56));
/* uaccess to a[16] expected to fail */
bytes = write(fd, a + 14, 8);
The iov_iter_fault_in_readable() check succeeds since a[14] has tag 0.
However, the copy_from_user() attempts an unaligned 8-byte load which
fails because of the mismatched tag from a[16]. The loop continues
indefinitely.
copy_from_user() is not required to squeeze in as much as possible. So I
think the 1-byte read per page via iov_iter_fault_in_readable() is not
sufficient to guarantee progress unless copy_from_user() also reads at
least 1 byte.
We could change raw_copy_from_user() to fall back to 1-byte read in case
of a fault or fix this corner case in the generic code. A quick hack,
re-attempting the access with one byte:
------------------8<-------------------------
diff --git a/mm/filemap.c b/mm/filemap.c
index 66f7e9fdfbc4..67059071460c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3686,8 +3686,18 @@ ssize_t generic_perform_write(struct file *file,
* because not all segments in the iov can be copied at
* once without a pagefault.
*/
- bytes = min_t(unsigned long, PAGE_SIZE - offset,
- iov_iter_single_seg_count(i));
+ unsigned long single_seg_bytes =
+ min_t(unsigned long, PAGE_SIZE - offset,
+ iov_iter_single_seg_count(i));
+
+ /*
+ * Check for intra-page faults (arm64 MTE, SPARC ADI)
+ * and fall back to single byte.
+ */
+ if (bytes > single_seg_bytes)
+ bytes = single_seg_bytes;
+ else
+ bytes = 1;
goto again;
}
pos += copied;
------------------8<-------------------------
Or a slightly different hack, trying to detect if the first segment was
crossing a page boundary:
------------------8<-------------------------
diff --git a/mm/filemap.c b/mm/filemap.c
index 66f7e9fdfbc4..7d1c03f5f559 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3678,16 +3678,24 @@ ssize_t generic_perform_write(struct file *file,
iov_iter_advance(i, copied);
if (unlikely(copied == 0)) {
+ struct iovec v = iov_iter_iovec(i);
+
/*
* If we were unable to copy any data at all, we must
- * fall back to a single segment length write.
+ * fall back to a single segment length write or a
+ * single byte write (for intra-page faults - arm64
+ * MTE or SPARC ADI).
*
* If we didn't fallback here, we could livelock
- * because not all segments in the iov can be copied at
- * once without a pagefault.
+ * because not all segments in the iov or data within
+ * a segment can be copied at once without a fault.
*/
- bytes = min_t(unsigned long, PAGE_SIZE - offset,
- iov_iter_single_seg_count(i));
+ if (((unsigned long)v.iov_base & PAGE_MASK) ==
+ ((unsigned long)(v.iov_base + bytes) & PAGE_MASK))
+ bytes = 1;
+ else
+ bytes = min_t(unsigned long, PAGE_SIZE - offset,
+ iov_iter_single_seg_count(i));
goto again;
}
pos += copied;
------------------8<-------------------------
--
Catalin
