ddbd89deb7
The problem I'm addressing was discovered by the LTP test covering
cve-2018-1000204.
A short description of what happens follows:
1) The test case issues a command code 00 (TEST UNIT READY) via the SG_IO
interface with: dxfer_len == 524288, dxdfer_dir == SG_DXFER_FROM_DEV
and a corresponding dxferp. The peculiar thing about this is that TUR
is not reading from the device.
2) In sg_start_req() the invocation of blk_rq_map_user() effectively
bounces the user-space buffer. As if the device was to transfer into
it. Since commit a45b599ad8 ("scsi: sg: allocate with __GFP_ZERO in
sg_build_indirect()") we make sure this first bounce buffer is
allocated with GFP_ZERO.
3) For the rest of the story we keep ignoring that we have a TUR, so the
device won't touch the buffer we prepare as if the we had a
DMA_FROM_DEVICE type of situation. My setup uses a virtio-scsi device
and the buffer allocated by SG is mapped by the function
virtqueue_add_split() which uses DMA_FROM_DEVICE for the "in" sgs (here
scatter-gather and not scsi generics). This mapping involves bouncing
via the swiotlb (we need swiotlb to do virtio in protected guest like
s390 Secure Execution, or AMD SEV).
4) When the SCSI TUR is done, we first copy back the content of the second
(that is swiotlb) bounce buffer (which most likely contains some
previous IO data), to the first bounce buffer, which contains all
zeros. Then we copy back the content of the first bounce buffer to
the user-space buffer.
5) The test case detects that the buffer, which it zero-initialized,
ain't all zeros and fails.
One can argue that this is an swiotlb problem, because without swiotlb
we leak all zeros, and the swiotlb should be transparent in a sense that
it does not affect the outcome (if all other participants are well
behaved).
Copying the content of the original buffer into the swiotlb buffer is
the only way I can think of to make swiotlb transparent in such
scenarios. So let's do just that if in doubt, but allow the driver
to tell us that the whole mapped buffer is going to be overwritten,
in which case we can preserve the old behavior and avoid the performance
impact of the extra bounce.
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
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DMA attributes
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==============
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This document describes the semantics of the DMA attributes that are
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defined in linux/dma-mapping.h.
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DMA_ATTR_WEAK_ORDERING
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----------------------
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DMA_ATTR_WEAK_ORDERING specifies that reads and writes to the mapping
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may be weakly ordered, that is that reads and writes may pass each other.
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Since it is optional for platforms to implement DMA_ATTR_WEAK_ORDERING,
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those that do not will simply ignore the attribute and exhibit default
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behavior.
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DMA_ATTR_WRITE_COMBINE
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----------------------
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DMA_ATTR_WRITE_COMBINE specifies that writes to the mapping may be
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buffered to improve performance.
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Since it is optional for platforms to implement DMA_ATTR_WRITE_COMBINE,
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those that do not will simply ignore the attribute and exhibit default
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behavior.
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DMA_ATTR_NO_KERNEL_MAPPING
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--------------------------
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DMA_ATTR_NO_KERNEL_MAPPING lets the platform to avoid creating a kernel
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virtual mapping for the allocated buffer. On some architectures creating
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such mapping is non-trivial task and consumes very limited resources
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(like kernel virtual address space or dma consistent address space).
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Buffers allocated with this attribute can be only passed to user space
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by calling dma_mmap_attrs(). By using this API, you are guaranteeing
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that you won't dereference the pointer returned by dma_alloc_attr(). You
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can treat it as a cookie that must be passed to dma_mmap_attrs() and
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dma_free_attrs(). Make sure that both of these also get this attribute
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set on each call.
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Since it is optional for platforms to implement
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DMA_ATTR_NO_KERNEL_MAPPING, those that do not will simply ignore the
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attribute and exhibit default behavior.
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DMA_ATTR_SKIP_CPU_SYNC
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----------------------
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By default dma_map_{single,page,sg} functions family transfer a given
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buffer from CPU domain to device domain. Some advanced use cases might
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require sharing a buffer between more than one device. This requires
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having a mapping created separately for each device and is usually
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performed by calling dma_map_{single,page,sg} function more than once
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for the given buffer with device pointer to each device taking part in
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the buffer sharing. The first call transfers a buffer from 'CPU' domain
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to 'device' domain, what synchronizes CPU caches for the given region
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(usually it means that the cache has been flushed or invalidated
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depending on the dma direction). However, next calls to
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dma_map_{single,page,sg}() for other devices will perform exactly the
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same synchronization operation on the CPU cache. CPU cache synchronization
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might be a time consuming operation, especially if the buffers are
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large, so it is highly recommended to avoid it if possible.
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DMA_ATTR_SKIP_CPU_SYNC allows platform code to skip synchronization of
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the CPU cache for the given buffer assuming that it has been already
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transferred to 'device' domain. This attribute can be also used for
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dma_unmap_{single,page,sg} functions family to force buffer to stay in
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device domain after releasing a mapping for it. Use this attribute with
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care!
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DMA_ATTR_FORCE_CONTIGUOUS
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-------------------------
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By default DMA-mapping subsystem is allowed to assemble the buffer
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allocated by dma_alloc_attrs() function from individual pages if it can
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be mapped as contiguous chunk into device dma address space. By
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specifying this attribute the allocated buffer is forced to be contiguous
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also in physical memory.
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DMA_ATTR_ALLOC_SINGLE_PAGES
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---------------------------
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This is a hint to the DMA-mapping subsystem that it's probably not worth
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the time to try to allocate memory to in a way that gives better TLB
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efficiency (AKA it's not worth trying to build the mapping out of larger
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pages). You might want to specify this if:
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- You know that the accesses to this memory won't thrash the TLB.
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You might know that the accesses are likely to be sequential or
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that they aren't sequential but it's unlikely you'll ping-pong
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between many addresses that are likely to be in different physical
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pages.
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- You know that the penalty of TLB misses while accessing the
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memory will be small enough to be inconsequential. If you are
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doing a heavy operation like decryption or decompression this
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might be the case.
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- You know that the DMA mapping is fairly transitory. If you expect
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the mapping to have a short lifetime then it may be worth it to
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optimize allocation (avoid coming up with large pages) instead of
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getting the slight performance win of larger pages.
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Setting this hint doesn't guarantee that you won't get huge pages, but it
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means that we won't try quite as hard to get them.
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.. note:: At the moment DMA_ATTR_ALLOC_SINGLE_PAGES is only implemented on ARM,
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though ARM64 patches will likely be posted soon.
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DMA_ATTR_NO_WARN
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----------------
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This tells the DMA-mapping subsystem to suppress allocation failure reports
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(similarly to __GFP_NOWARN).
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On some architectures allocation failures are reported with error messages
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to the system logs. Although this can help to identify and debug problems,
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drivers which handle failures (eg, retry later) have no problems with them,
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and can actually flood the system logs with error messages that aren't any
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problem at all, depending on the implementation of the retry mechanism.
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So, this provides a way for drivers to avoid those error messages on calls
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where allocation failures are not a problem, and shouldn't bother the logs.
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.. note:: At the moment DMA_ATTR_NO_WARN is only implemented on PowerPC.
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DMA_ATTR_PRIVILEGED
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-------------------
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Some advanced peripherals such as remote processors and GPUs perform
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accesses to DMA buffers in both privileged "supervisor" and unprivileged
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"user" modes. This attribute is used to indicate to the DMA-mapping
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subsystem that the buffer is fully accessible at the elevated privilege
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level (and ideally inaccessible or at least read-only at the
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lesser-privileged levels).
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DMA_ATTR_OVERWRITE
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------------------
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This is a hint to the DMA-mapping subsystem that the device is expected to
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overwrite the entire mapped size, thus the caller does not require any of the
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previous buffer contents to be preserved. This allows bounce-buffering
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implementations to optimise DMA_FROM_DEVICE transfers.
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