[tip:efi/urgent] efi: Fix for_each_efi_memory_desc_in_map() for empty memmaps

[tip-bot for Vitaly Kuznetsov]

Commit-ID:  55f1ea15216a5a14c96738bd5284100a00ffa9dc
Gitweb:     http://git.kernel.org/tip/55f1ea15216a5a14c96738bd5284100a00ffa9dc
Author:     Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
AuthorDate: Tue, 31 May 2016 11:23:43 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 3 Jun 2016 09:57:35 +0200

efi: Fix for_each_efi_memory_desc_in_map() for empty memmaps

Commit:

  78ce248faa3c ("efi: Iterate over efi.memmap in for_each_efi_memory_desc()")

introduced a regression for systems booted with the 'noefi' kernel option.

In particular, I observed an early kernel hang in efi_find_mirror()'s
for_each_efi_memory_desc() call. As we don't have efi memmap on this
system we enter this iterator with the following parameters:

  efi.memmap.map = 0, efi.memmap.map_end = 0, efi.memmap.desc_size = 28

... then for_each_efi_memory_desc_in_map() does the following comparison:

  (md) <= (efi_memory_desc_t *)((m)->map_end - (m)->desc_size);

... where md = 0, (m)->map_end = 0 and (m)->desc_size = 28 but when we subtract
something from a NULL pointer wrap around happens and we end up returning
invalid pointer and crash.

Fix it by using the correct pointer arithmetics.

Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Signed-off-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mark Salter <msalter@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: linux-efi@xxxxxxxxxxxxxxx
Fixes: 78ce248faa3c ("efi: Iterate over efi.memmap in for_each_efi_memory_desc()")
Link: http://lkml.kernel.org/r/1464690224-4503-2-git-send-email-matt@xxxxxxxxxxxxxxxxxxx
[ Made the changelog more readable. ]
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 include/linux/efi.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/efi.h b/include/linux/efi.h
index c2db3ca..f196dd0 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -1005,7 +1005,7 @@ extern int efi_memattr_apply_permissions(struct mm_struct *mm,
 /* Iterate through an efi_memory_map */
 #define for_each_efi_memory_desc_in_map(m, md)				   \
 	for ((md) = (m)->map;						   \
-	     (md) <= (efi_memory_desc_t *)((m)->map_end - (m)->desc_size); \
+	     ((void *)(md) + (m)->desc_size) <= (m)->map_end;		   \
 	     (md) = (void *)(md) + (m)->desc_size)
 
 /**