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Date: Thu, 26 May 2016 11:26:37 +0200 From: Vitaly Kuznetsov <vkuznets@...hat.com> To: linux-efi@...r.kernel.org Cc: linux-kernel@...r.kernel.org, Matt Fleming <matt@...eblueprint.co.uk>, Mark Salter <msalter@...hat.com>, "K. Y. Srinivasan" <kys@...rosoft.com> Subject: [PATCH v2] 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 'noefi' kernel option. In particular, I observe early kernel hang in efi_find_mirror() on for_each_efi_memory_desc() call. As we don't have efi memmap we enter this iterator with the following parameters: efi.memmap.map = 0, efi.memmap.map_end = 0, efi.memmap.desc_size = 28 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. Fixes: 78ce248faa3c ("efi: Iterate over efi.memmap in for_each_efi_memory_desc()") Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com> --- Changes since v1: cast to (void *) instead of (efi_memory_desc_t *) [Matt Fleming]. It seems we're still exercising some sort of an undefined behavioir as C standard only allows us to compare pointers which point to the same object in memory (or one position past the end of the object) and I doubt NULL pointers are covered by this rule but gcc gives no warning, I think we're relatively safe. It may also make sense to write this comparison casting everything to something like unsigned long I guess... --- 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) /** -- 2.5.5
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