[PATCH] efi_loader: fix pe reloc pointer overrun
Heinrich Schuchardt
xypron.glpk at gmx.de
Thu Dec 5 10:52:40 CET 2024
On 05.12.24 09:58, Aleksandar Gerasimovski wrote:
>> If VirtualSize = 16, then the reloc section has 16 bytes and not 12.
>> A section size of less than 512 is invalid anyway.
>> Something is wrong with the tooling that you use to create the EFI binary.
> This is valid point, we will follow that track as well, but at the end is a boot-guard
> efi binary.
>
>> typedef struct _IMAGE_BASE_RELOCATION
>> {
>> uint32_t VirtualAddress;
>> uint32_t SizeOfBlock; >
>> /* WORD TypeOffset[1]; */
>> } IMAGE_BASE_RELOCATION,*PIMAGE_BASE_RELOCATION;
>
>> The loop stops if rel->SizeOfBlock == 0.
>
> Correct, but look how the rel pointer is moving for 2 bytes:
> const uint16_t *relocs = (const uint16_t *)(rel + 1);
> ...
> rel = (const IMAGE_BASE_RELOCATION *)relocs;
>
> And it ends at bce2600c, and then takes 7c00 9340 for size ; )
>
>> "two relocations at offset bce26008 and bce2600a" cannot be true.
>
> This I really don't know, yes we need to follow with the tolling.
> In any case this image however is generated is let to run to the point when
> it break the u-boot, the image must be at least rejected earlier.
>
> Regards,
> Aleksandar
>
> From: Heinrich Schuchardt <xypron.glpk at gmx.de>
> Sent: Donnerstag, 5. Dezember 2024 09:35
> To: Aleksandar Gerasimovski <Aleksandar.Gerasimovski at belden.com>
> Cc: u-boot at lists.denx.de; Ilias Apalodimas <ilias.apalodimas at linaro.org>
> Subject: Re: [PATCH] efi_loader: fix pe reloc pointer overrun
>
> On 05.12.24 09:13, Aleksandar Gerasimovski wrote:
>> Thanks for the replay,
>>
>>> Sections have to be at least 512 bytes aligned. The file alignment
>>> cannot be smaller the 512 bytes either.
>>
>>> Hence for a valid EFI binary the value of "rel" when entering
>>> efi_loader_relocate() will always be at least 512 aligned.
>>
>> That's not what I see in the case that I try to explain, because
>> the sec->Misc.VirtualSize virtual size is 16 bytes for this section .
>>
>>> The situation above cannot occur if you follow the specification and
>>> zero fill the raw data up to SizeOfRawData which is a multiple of 512.
>>
>> Correct, I see in the efi_load_pe code that section will be zeroed
>> If sec->Misc.VirtualSize is bigger then sec->SizeOfRawData, but
>> this is not a case for our image it's oposite.
>> I can also confirm that in the real image the section is zeroed up to
>> sec->SizeOfRawData, but the problem is that only the sec->Misc.VirtualSize
>> (that is 16 bytes for my image) is copied to the "efi_reloc" location.
>>
>> I'm far from really understanding the PE format, actually this is first
>> time to deal with this and efi boot on Linux, however our EFI image is
>> generated with a "standard" tools like bg_gen_unified_kernel by our
>> yocto based distro generator, so in any case I see a nasty bug.
>> Even if the image is wrong it should be rejected earlier and not overflowing
>> even overwriting the u-boot code. It's a vulnerability.
>>
>>> Please, provide an example image for download to give the full picture.
>>
>> I must think how to do that, where I could upload?
>>
>>> With "rel + 1 < end" you are not preserving the change but reverting it.
>>
>> Well, this is the tricky part, in my understanding not because rel is moving
>> by 2 bytes down in the loop.
>>
>> I'm trying to describe the problem again, let's say in my image the problematic section
>> is generated with sec->Misc.VirtualSize = 16 and sec->SizeOfRawData = 512, what will be
>> provided to the efi_loader_relocate in this case is rel_size of 16 and only 16 bytes will
>> be copied to efi_reloc location (see the " Load sections into RAM" logic in efi_load_pe),
>> so the content of the RAM after efi_reloc + 16 stays unknown:
>> bce26000: 3000 0000 000c 0000 0000 0000 0000 0000 -> reloc section
>> bce26010: 7c00 9340 67e0 f900 1c00 0ea1 a400 0f20 -> random content
>>
>> So efi_loader_relocate gets rel_size of 16 bytes for reloc section of 12 bytes,
>> and this will overflow.
>
> If VirtualSize = 16, then the reloc section has 16 bytes and not 12.
>
> A section size of less than 512 is invalid anyway.
>
> Something is wrong with the tooling that you use to create the EFI binary.
>
>>
>> When I'm writing this, I'm thinking isn't then rel_size problem as well,
>> that is 16 bytes in this case?
>>
>> From: Heinrich Schuchardt <mailto:xypron.glpk at gmx.de>
>> Sent: Mittwoch, 4. Dezember 2024 11:11
>> To: Aleksandar Gerasimovski <mailto:Aleksandar.Gerasimovski at belden.com>
>> Cc: mailto:u-boot at lists.denx.de; Ilias Apalodimas <mailto:ilias.apalodimas at linaro.org>
>> Subject: Re: [PATCH] efi_loader: fix pe reloc pointer overrun
>>
>>
>> On 29.11.24 22:09, Aleksandar Gerasimovski wrote:
>>> The fix provided by 997fc12ec91 is actually introducing
>>> a buffer overrun, and the overrun is effective if the
>>> memory after the reloc section is not zeroed.
>>> Probably that's why this bug is not always noticeable.
>>>
>>> The problem is that 8-bytes 'rel' pointer can be 4-bytes aligned
>>> according to the PE Format, so the actual relocate function can
>>> take values after the reloc section.
>>
>> Sections have to be at least 512 bytes aligned. The file alignment
>> cannot be smaller the 512 bytes either.
>>
>> Hence for a valid EFI binary the value of "rel" when entering
>> efi_loader_relocate() will always be at least 512 aligned.
>>
>>>
>>> One example is the following dump from the reloc section:
>>> bce26000: 3000 0000 000c 0000 0000 0000 0000 0000
>>> bce26010: 7c00 9340 67e0 f900 1c00 0ea1 a400 0f20
>>
>> Please, provide an example image for download to give the full picture.
>>
>>>
>>> This section has two relocations at offset bce26008 and bce2600a,
>
> Starting at bce26008 you have eight zero bytes with this structure:
>
> typedef struct _IMAGE_BASE_RELOCATION
> {
> uint32_t VirtualAddress;
> uint32_t SizeOfBlock;
> /* WORD TypeOffset[1]; */
> } IMAGE_BASE_RELOCATION,*PIMAGE_BASE_RELOCATION;
>
> The loop stops if rel->SizeOfBlock == 0.
>
> "two relocations at offset bce26008 and bce2600a" cannot be true.
>
> Best regards
>
> Heinrich
>
>>> however the given size (rel_size) for this relocation is 16-bytes
>>
>> The value of "rel_size" used to calculate "end" is not the size of a
>> relocation but the size of the relocation table.
>>
>> The PE-COFF specification describes:
>>
>> "SizeOfRawData
>>
>> The size of the section (for object files) or the size of the
>> initialized data on disk (for image files). For executable images, this
>> must be a multiple of FileAlignment from the optional header. If this is
>> less than VirtualSize, the remainder of the section is zero-filled."
>>
>> The situation above cannot occur if you follow the specification and
>> zero fill the raw data up to SizeOfRawData which is a multiple of 512.
>>
>>> and this is coming form the efi image Misc.VirtualSize, so in this
>>> case the 'reloc' pointer ends at affset bce2600c and is taken as
>>> valid and this is where the overflow is.
>>>
>>> In our system we see this problem when we are starting the
>>> Boot Guard efi image.
>>>
>>> This patch is fixing the overrun while preserving the fix done
>>> by 997fc12ec91.
>>
>> Before 997fc12ec91 we had "rel < end - 1".
>>
>> With "rel + 1 < end" you are not preserving the change but reverting it.
>>
const uint16_t *relocs = (const uint16_t *)(rel + 1);
points to the first relocation of the relocation block.
rel + 1 must be inside the relocation section or it cannot point to a
valid relocation.
In the line
i = (rel->SizeOfBlock - sizeof(*rel)) / sizeof(uint16_t);
we assume but do not check that
(u8 *)rel + rel->SizeOfBlock <= end
and
rel->SizeOfBlock >= sizeof(*rel)
A malformed relocation section could still lead to a buffer overrun.
Fixes: 997fc12ec91e ("efi_loader: do not miss last relocation block")
Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
>>
>>>
>>> Signed-off-by: Aleksandar Gerasimovski <mailto:aleksandar.gerasimovski at belden.com>
>>> ---
>>> lib/efi_loader/efi_image_loader.c | 2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
>>> index 0ddf69a091..04075d83cb 100644
>>> --- a/lib/efi_loader/efi_image_loader.c
>>> +++ b/lib/efi_loader/efi_image_loader.c
>>> @@ -121,7 +121,7 @@ static efi_status_t efi_loader_relocate(const IMAGE_BASE_RELOCATION *rel,
>>> return EFI_SUCCESS;
>>>
>>> end = (const IMAGE_BASE_RELOCATION *)((const char *)rel + rel_size);
>>> - while (rel < end && rel->SizeOfBlock) {
>>> + while (rel + 1 < end && rel->SizeOfBlock) {
>>> const uint16_t *relocs = (const uint16_t *)(rel + 1);
>>> i = (rel->SizeOfBlock - sizeof(*rel)) / sizeof(uint16_t);
>>> while (i--) {
>>> --
>>> 2.34.1
>>>
>>> **********************************************************************
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>>
>>
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>
>
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