[PATCH 3.16.y-ckt 057/165] efi/libstub: Fix boundary checking in efi_high_alloc()

[Luis Henriques]

3.16.7-ckt9 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yinghai Lu <yinghai@xxxxxxxxxx>

commit 7ed620bb343f434f8a85f830020c04988df2a140 upstream.

While adding support loading kernel and initrd above 4G to grub2 in legacy
mode, I was referring to efi_high_alloc().
That will allocate buffer for kernel and then initrd, and initrd will
use kernel buffer start as limit.

During testing found two buffers will be overlapped when initrd size is
very big like 400M.

It turns out efi_high_alloc() boundary checking is not right.
end - size will be the new start, and should not compare new
start with max, we need to make sure end is smaller than max.

[ Basically, with the current efi_high_alloc() code it's possible to
  allocate memory above 'max', because efi_high_alloc() doesn't check
  that the tail of the allocation is below 'max'.

  If you have an EFI memory map with a single entry that looks like so,

   [0xc0000000-0xc0004000]

  And want to allocate 0x3000 bytes below 0xc0003000 the current code
  will allocate [0xc0001000-0xc0004000], not [0xc0000000-0xc0003000]
  like you would expect. - Matt ]

Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Tested-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Matt Fleming <matt.fleming@xxxxxxxxx>
[ luis: backported to 3.16:
  - file rename: drivers/firmware/efi/libstub/efi-stub-helper.c ->
    drivers/firmware/efi/efi-stub-helper.c ]
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 drivers/firmware/efi/efi-stub-helper.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/firmware/efi/efi-stub-helper.c b/drivers/firmware/efi/efi-stub-helper.c
index eb6d4be9e722..2fb3a0aab56c 100644
--- a/drivers/firmware/efi/efi-stub-helper.c
+++ b/drivers/firmware/efi/efi-stub-helper.c
@@ -157,12 +157,12 @@ again:
 		start = desc->phys_addr;
 		end = start + desc->num_pages * (1UL << EFI_PAGE_SHIFT);
 
-		if ((start + size) > end || (start + size) > max)
-			continue;
-
-		if (end - size > max)
+		if (end > max)
 			end = max;
 
+		if ((start + size) > end)
+			continue;
+
 		if (round_down(end - size, align) < start)
 			continue;
 
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