[PATCH] efi/libstub: fix efi_random_alloc() to allocate memory at alloc_min or higher address

[KONDO KAZUMA(近藤 和真)]

Following warning is sometimes observed while booting my servers:
  [    3.594838] DMA: preallocated 4096 KiB GFP_KERNEL pool for atomic allocations
  [    3.602918] swapper/0: page allocation failure: order:10, mode:0xcc1(GFP_KERNEL|GFP_DMA), nodemask=(null),cpuset=/,mems_allowed=0-1
  ...
  [    3.851862] DMA: preallocated 1024 KiB GFP_KERNEL|GFP_DMA pool for atomic allocation

If 'nokaslr' boot option is set, the warning always happens.

On x86, ZONE_DMA is small zone at the first 16MB of physical address
space. When this problem happens, most of that space seems to be used
by decompressed kernel. Thereby, there is not enough space at DMA_ZONE
to meet the request of DMA pool allocation.

The commit 2f77465b05b1 ("x86/efistub: Avoid placing the kernel below LOAD_PHYSICAL_ADDR")
tried to fix this problem by introducing lower bound of allocation.

But the fix is not complete.

efi_random_alloc() allocates pages by following steps.
1. Count total available slots ('total_slots')
2. Select a slot ('target_slot') to allocate randomly
3. Calculate a starting address ('target') to be included target_slot
4. Allocate pages, which starting address is 'target'

In step 1, 'alloc_min' is used to offset the starting address of
memory chunk. But in step 3 'alloc_min' is not considered at all.
As the result, 'target' can be miscalculated and become lower
than 'alloc_min'.

When KASLR is disabled, 'target_slot' is always 0 and
the problem happens everytime if the EFI memory map of the system
meets the condition.

Fix this problem by calculating 'target' considering 'alloc_min'.

Cc: linux-efi@xxxxxxxxxxxxxxx
Cc: Tom Englund <tomenglund26@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Fixes: 2f77465b05b1 ("x86/efistub: Avoid placing the kernel below LOAD_PHYSICAL_ADDR")
Signed-off-by: Kazuma Kondo <kazuma-kondo@xxxxxxx>
---
 drivers/firmware/efi/libstub/randomalloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/randomalloc.c b/drivers/firmware/efi/libstub/randomalloc.c
index 4e96a855fdf4..7e1852859550 100644
--- a/drivers/firmware/efi/libstub/randomalloc.c
+++ b/drivers/firmware/efi/libstub/randomalloc.c
@@ -120,7 +120,7 @@ efi_status_t efi_random_alloc(unsigned long size,
 			continue;
 		}
 
-		target = round_up(md->phys_addr, align) + target_slot * align;
+		target = round_up(max(md->phys_addr, alloc_min), align) + target_slot * align;
 		pages = size / EFI_PAGE_SIZE;
 
 		status = efi_bs_call(allocate_pages, EFI_ALLOCATE_ADDRESS,
-- 
2.39.3