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drm/i915: Prefault before locking pages in shmem_pwrite

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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: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401133909.31203-1-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson 2019-04-01 14:39:09 +01:00
parent d53fef0be4
commit b01720bfcd
1 changed files with 24 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

30
drivers/gpu/drm/i915/i915_gem.c
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@ -2788,7 +2788,11 @@ i915_gem_object_pwrite_gtt(struct drm_i915_gem_object *obj,
u64 remain, offset;
unsigned int pg;
/* Before we instantiate/pin the backing store for our use, we
/* Caller already validated user args */
GEM_BUG_ON(!access_ok(user_data, arg->size));
/*
* Before we instantiate/pin the backing store for our use, we
* can prepopulate the shmemfs filp efficiently using a write into
* the pagecache. We avoid the penalty of instantiating all the
* pages, important if the user is just writing to a few and never
@ -2802,7 +2806,8 @@ i915_gem_object_pwrite_gtt(struct drm_i915_gem_object *obj,
if (obj->mm.madv != I915_MADV_WILLNEED)
return -EFAULT;
/* Before the pages are instantiated the object is treated as being
/*
* Before the pages are instantiated the object is treated as being
* in the CPU domain. The pages will be clflushed as required before
* use, and we can freely write into the pages directly. If userspace
* races pwrite with any other operation; corruption will ensue -
@ -2818,20 +2823,32 @@ i915_gem_object_pwrite_gtt(struct drm_i915_gem_object *obj,
struct page *page;
void *data, *vaddr;
int err;
char c;
len = PAGE_SIZE - pg;
if (len > remain)
len = remain;
/* Prefault the user page to reduce potential recursion */
err = __get_user(c, user_data);
if (err)
return err;
err = __get_user(c, user_data + len - 1);
if (err)
return err;
err = pagecache_write_begin(obj->base.filp, mapping,
offset, len, 0,
&page, &data);
if (err < 0)
return err;
vaddr = kmap(page);
unwritten = copy_from_user(vaddr + pg, user_data, len);
kunmap(page);
vaddr = kmap_atomic(page);
unwritten = __copy_from_user_inatomic(vaddr + pg,
user_data,
len);
kunmap_atomic(vaddr);
err = pagecache_write_end(obj->base.filp, mapping,
offset, len, len - unwritten,
@ -2839,8 +2856,9 @@ i915_gem_object_pwrite_gtt(struct drm_i915_gem_object *obj,
if (err < 0)
return err;
/* We don't handle -EFAULT, leave it to the caller to check */
if (unwritten)
return -EFAULT;
return -ENODEV;
remain -= len;
user_data += len;