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

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 2f77465b05 ("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@vger.kernel.org
Cc: Tom Englund <tomenglund26@gmail.com>
Cc: linux-kernel@vger.kernel.org
Fixes: 2f77465b05 ("x86/efistub: Avoid placing the kernel below LOAD_PHYSICAL_ADDR")
Signed-off-by: Kazuma Kondo <kazuma-kondo@nec.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

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,