[PATCH 1/1] fbdev/efifb: ignore framebuffer memmap entries that lack any memory types

[Ard Biesheuvel]

Commit 38ac0287b7f4

  ("fbdev/efifb: Honour UEFI memory map attributes when mapping the FB")

updated the EFI framebuffer code to use memory mappings for the linear
framebuffer that are permitted by the memory attributes described by the
EFI memory map for the particular region, if the framebuffer happens to
be covered by the EFI memory map (which is typically only the case for
framebuffers in shared memory). This is required since non-X86 systems
may require cacheable attributes for memory mappings that are shared
with other masters (such as GPUs), and this information cannot be
described by the Graphics Output Protocol (GOP) EFI protocol itself,
and so we rely on the EFI memory map for this.

As reported by James, this breaks some x86 systems:

  [ 1.173368] efifb: probing for efifb
  [ 1.173386] efifb: abort, cannot remap video memory 0x1d5000 @ 0xcf800000
  [ 1.173395] Trying to free nonexistent resource <00000000cf800000-00000000cf9d4bff>
  [ 1.173413] efi-framebuffer: probe of efi-framebuffer.0 failed with error -5

The problem turns out to be that the memory map entry that describes the
framebuffer has no memory attributes listed at all, and so we end up with
a mem_flags value of 0x0.

So work around this by ensuring that the memory map entry's attribute field
has a sane value before using it to mask the set of usable attributes.

Fixes: 38ac0287b7f4 ("fbdev/efifb: Honour UEFI memory map attributes when ...")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.19+
Cc: Peter Jones <pjones@xxxxxxxxxx>
Reported-by: James Hilliard <james.hilliard1@xxxxxxxxx>
Tested-by: James Hilliard <james.hilliard1@xxxxxxxxx>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
 drivers/video/fbdev/efifb.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/video/fbdev/efifb.c b/drivers/video/fbdev/efifb.c
index 9e529cc2b4ff..9f39f0c360e0 100644
--- a/drivers/video/fbdev/efifb.c
+++ b/drivers/video/fbdev/efifb.c
@@ -477,8 +477,12 @@ static int efifb_probe(struct platform_device *dev)
 		 * If the UEFI memory map covers the efifb region, we may only
 		 * remap it using the attributes the memory map prescribes.
 		 */
-		mem_flags |= EFI_MEMORY_WT | EFI_MEMORY_WB;
-		mem_flags &= md.attribute;
+		md.attribute &= EFI_MEMORY_UC | EFI_MEMORY_WC |
+				EFI_MEMORY_WT | EFI_MEMORY_WB;
+		if (md.attribute) {
+			mem_flags |= EFI_MEMORY_WT | EFI_MEMORY_WB;
+			mem_flags &= md.attribute;
+		}
 	}
 	if (mem_flags & EFI_MEMORY_WC)
 		info->screen_base = ioremap_wc(efifb_fix.smem_start,
-- 
2.20.1