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drm/i915: Prefault before locking pages in shmem_pwrite
If the user passes in a pointer to a GGTT mmaping of the same buffer being written to, we can hit a deadlock in acquiring the shmemfs page (once as the write destination and then as the read source). [<0>] io_schedule+0xd/0x30 [<0>] __lock_page+0x105/0x1b0 [<0>] find_lock_entry+0x55/0x90 [<0>] shmem_getpage_gfp+0xbb/0x800 [<0>] shmem_read_mapping_page_gfp+0x2d/0x50 [<0>] shmem_get_pages+0x158/0x5d0 [i915] [<0>] ____i915_gem_object_get_pages+0x17/0x90 [i915] [<0>] __i915_gem_object_get_pages+0x57/0x70 [i915] [<0>] i915_gem_fault+0x1b4/0x5c0 [i915] [<0>] __do_fault+0x2d/0x80 [<0>] __handle_mm_fault+0xad4/0xfb0 [<0>] handle_mm_fault+0xe6/0x1f0 [<0>] __do_page_fault+0x18f/0x3f0 [<0>] page_fault+0x1b/0x20 [<0>] copy_user_enhanced_fast_string+0x7/0x10 [<0>] _copy_from_user+0x37/0x60 [<0>] shmem_pwrite+0xf0/0x160 [i915] [<0>] i915_gem_pwrite_ioctl+0x14e/0x520 [i915] [<0>] drm_ioctl_kernel+0x81/0xd0 [<0>] drm_ioctl+0x1a7/0x310 [<0>] do_vfs_ioctl+0x88/0x5d0 [<0>] ksys_ioctl+0x35/0x70 [<0>] __x64_sys_ioctl+0x11/0x20 [<0>] do_syscall_64+0x39/0xe0 [<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9 We can reduce (but not eliminate!) the chance of this happening by faulting the user_data before we take the page lock in pagecache_write_begin(). One way to eliminate the potential recursion here is by disabling pagefaults for the copy, and handling the fallback to use an alternative method -- so convert to use kmap_atomic (which should disable preemption and pagefaulting for the copy) and report ENODEV instead of EFAULT so that our caller tries again with a different copy mechanism -- we already check that the page should have been faultable so a false negative should be rare. Testcase: igt/gem_pwrite/self Signed-off-by:Chris Wilson <chris@chris-wilson.co.uk> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Cc: Matthew Auld <matthew.william.auld@gmail.com> Reviewed-by: