Messages in this thread | | | Date | Wed, 7 Oct 2020 14:27:46 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH 07/13] mm: close race in generic_access_phys |
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On Wed, Oct 07, 2020 at 06:44:20PM +0200, Daniel Vetter wrote: > Way back it was a reasonable assumptions that iomem mappings never > change the pfn range they point at. But this has changed: > > - gpu drivers dynamically manage their memory nowadays, invalidating > ptes with unmap_mapping_range when buffers get moved > > - contiguous dma allocations have moved from dedicated carvetouts to > cma regions. This means if we miss the unmap the pfn might contain > pagecache or anon memory (well anything allocated with GFP_MOVEABLE) > > - even /dev/mem now invalidates mappings when the kernel requests that > iomem region when CONFIG_IO_STRICT_DEVMEM is set, see 3234ac664a87 > ("/dev/mem: Revoke mappings when a driver claims the region") > > Accessing pfns obtained from ptes without holding all the locks is > therefore no longer a good idea. Fix this. > > Since ioremap might need to manipulate pagetables too we need to drop > the pt lock and have a retry loop if we raced. > > While at it, also add kerneldoc and improve the comment for the > vma_ops->access function. It's for accessing, not for moving the > memory from iomem to system memory, as the old comment seemed to > suggest. > > References: 28b2ee20c7cb ("access_process_vm device memory infrastructure") > Cc: Jason Gunthorpe <jgg@ziepe.ca> > Cc: Dan Williams <dan.j.williams@intel.com> > Cc: Kees Cook <keescook@chromium.org> > Cc: Rik van Riel <riel@redhat.com> > Cc: Benjamin Herrensmidt <benh@kernel.crashing.org> > Cc: Dave Airlie <airlied@linux.ie> > Cc: Hugh Dickins <hugh@veritas.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: John Hubbard <jhubbard@nvidia.com> > Cc: Jérôme Glisse <jglisse@redhat.com> > Cc: Jan Kara <jack@suse.cz> > Cc: Dan Williams <dan.j.williams@intel.com> > Cc: linux-mm@kvack.org > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-samsung-soc@vger.kernel.org > Cc: linux-media@vger.kernel.org > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com> > --- > include/linux/mm.h | 3 ++- > mm/memory.c | 44 ++++++++++++++++++++++++++++++++++++++++++-- > 2 files changed, 44 insertions(+), 3 deletions(-)
This does seem to solve the race with revoke_devmem(), but it is really ugly.
It would be much nicer to wrap a rwsem around this access and the unmap.
Any place using it has a nice linear translation from vm_off to pfn, so I don't think there is a such a good reason to use follow_pte in the first place.
ie why not the helper be this:
int generic_access_phys(unsigned long pfn, unsigned long pgprot, void *buf, size_t len, bool write)
Then something like dev/mem would compute pfn and obtain the lock:
dev_access(struct vm_area_struct *vma, unsigned long addr, void *buf, int len, int write) { cpu_addr = vma->vm_pgoff*PAGE_SIZE + (addr - vma->vm_start));
/* FIXME: Has to be over each page of len */ if (!devmem_is_allowed_access(PHYS_PFN(cpu_addr/4096))) return -EPERM;
down_read(&mem_sem); generic_access_phys(cpu_addr/4096, pgprot_val(vma->vm_page_prot), buf, len, write); up_read(&mem_sem); }
The other cases looked simpler because they don't revoke, here the mmap_sem alone should be enough protection, they would just need to provide the linear translation to pfn.
What do you think?
Jason
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