On Wed, Oct 27, 2021 at 01:40:47AM +0000, Chen Huang wrote:
From: Robin Murphy robin.murphy@arm.com
commit 295cf156231ca3f9e3a66bde7fab5e09c41835e0 upstream.
Al reminds us that the usercopy API must only return complete failure if absolutely nothing could be copied. Currently, if userspace does something silly like giving us an unaligned pointer to Device memory, or a size which overruns MTE tag bounds, we may fail to honour that requirement when faulting on a multi-byte access even though a smaller access could have succeeded.
Add a mitigation to the fixup routines to fall back to a single-byte copy if we faulted on a larger access before anything has been written to the destination, to guarantee making *some* forward progress. We needn't be too concerned about the overall performance since this should only occur when callers are doing something a bit dodgy in the first place. Particularly broken userspace might still be able to trick generic_perform_write() into an infinite loop by targeting write() at an mmap() of some read-only device register where the fault-in load succeeds but any store synchronously aborts such that copy_to_user() is genuinely unable to make progress, but, well, don't do that...
CC: stable@vger.kernel.org Reported-by: Chen Huang chenhuang5@huawei.com Suggested-by: Al Viro viro@zeniv.linux.org.uk Reviewed-by: Catalin Marinas catalin.marinas@arm.com Signed-off-by: Robin Murphy robin.murphy@arm.com Link: https://lore.kernel.org/r/dc03d5c675731a1f24a62417dba5429ad744234e.162609843... Signed-off-by: Will Deacon will@kernel.org Signed-off-by: Chen Huang chenhuang5@huawei.com
arch/arm64/lib/copy_from_user.S | 13 ++++++++++--- arch/arm64/lib/copy_in_user.S | 21 ++++++++++++++------- arch/arm64/lib/copy_to_user.S | 14 +++++++++++--- 3 files changed, 35 insertions(+), 13 deletions(-)
Both now queued up, thanks.
greg k-h