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: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190401133909.31203-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index c3b4ec5..bf594a5 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -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;