Re: [PATCH v7 bpf-next 01/10] lib/buildid: harden build ID parsing logic

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Aug 29, 2024 at 10:42:23AM -0700, Andrii Nakryiko wrote:
> Harden build ID parsing logic, adding explicit READ_ONCE() where it's
> important to have a consistent value read and validated just once.
> 
> Also, as pointed out by Andi Kleen, we need to make sure that entire ELF
> note is within a page bounds, so move the overflow check up and add an
> extra note_size boundaries validation.
> 
> Fixes tag below points to the code that moved this code into
> lib/buildid.c, and then subsequently was used in perf subsystem, making
> this code exposed to perf_event_open() users in v5.12+.
> 
> Cc: stable@vger.kernel.org
> Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
> Reviewed-by: Jann Horn <jannh@google.com>
> Suggested-by: Andi Kleen <ak@linux.intel.com>
> Fixes: bd7525dacd7e ("bpf: Move stack_map_get_build_id into lib")
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  lib/buildid.c | 76 +++++++++++++++++++++++++++++----------------------
>  1 file changed, 44 insertions(+), 32 deletions(-)
> 
> diff --git a/lib/buildid.c b/lib/buildid.c
> index e02b5507418b..26007cc99a38 100644
> --- a/lib/buildid.c
> +++ b/lib/buildid.c
> @@ -18,31 +18,37 @@ static int parse_build_id_buf(unsigned char *build_id,
>  			      const void *note_start,
>  			      Elf32_Word note_size)
>  {
> -	Elf32_Word note_offs = 0, new_offs;
> -
> -	while (note_offs + sizeof(Elf32_Nhdr) < note_size) {
> -		Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_offs);
> +	const char note_name[] = "GNU";
> +	const size_t note_name_sz = sizeof(note_name);
> +	u64 note_off = 0, new_off, name_sz, desc_sz;
> +	const char *data;
> +
> +	while (note_off + sizeof(Elf32_Nhdr) < note_size &&
> +	       note_off + sizeof(Elf32_Nhdr) > note_off /* overflow */) {
> +		Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_off);
> +
> +		name_sz = READ_ONCE(nhdr->n_namesz);
> +		desc_sz = READ_ONCE(nhdr->n_descsz);
> +
> +		new_off = note_off + sizeof(Elf32_Nhdr);
> +		if (check_add_overflow(new_off, ALIGN(name_sz, 4), &new_off) ||
> +		    check_add_overflow(new_off, ALIGN(desc_sz, 4), &new_off) ||
> +		    new_off > note_size)
> +			break;
>  
>  		if (nhdr->n_type == BUILD_ID &&
> -		    nhdr->n_namesz == sizeof("GNU") &&
> -		    !strcmp((char *)(nhdr + 1), "GNU") &&
> -		    nhdr->n_descsz > 0 &&
> -		    nhdr->n_descsz <= BUILD_ID_SIZE_MAX) {
> -			memcpy(build_id,
> -			       note_start + note_offs +
> -			       ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
> -			       nhdr->n_descsz);
> -			memset(build_id + nhdr->n_descsz, 0,
> -			       BUILD_ID_SIZE_MAX - nhdr->n_descsz);
> +		    name_sz == note_name_sz &&
> +		    memcmp(nhdr + 1, note_name, note_name_sz) == 0 &&
> +		    desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
> +			data = note_start + note_off + ALIGN(note_name_sz, 4);
> +			memcpy(build_id, data, desc_sz);
> +			memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
>  			if (size)
> -				*size = nhdr->n_descsz;
> +				*size = desc_sz;
>  			return 0;
>  		}

hi,
this fix is causing stable kernels to return wrong build id,
the change below seems to fix that (based on 6.6 stable)

if we agree on the fix I'll send it to all affected stable trees

jirka


---
The parse_build_id_buf does not account Elf32_Nhdr header size
when getting the build id data pointer and returns wrong build
id data as result.

This is problem only stable trees that merged c83a80d8b84f fix,
the upstream build id code was refactored and returns proper
build id.

Fixes: c83a80d8b84f ("lib/buildid: harden build ID parsing logic")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 lib/buildid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/buildid.c b/lib/buildid.c
index d3bc3d0528d5..9fc46366597e 100644
--- a/lib/buildid.c
+++ b/lib/buildid.c
@@ -40,7 +40,7 @@ static int parse_build_id_buf(unsigned char *build_id,
 		    name_sz == note_name_sz &&
 		    memcmp(nhdr + 1, note_name, note_name_sz) == 0 &&
 		    desc_sz > 0 && desc_sz <= BUILD_ID_SIZE_MAX) {
-			data = note_start + note_off + ALIGN(note_name_sz, 4);
+			data = note_start + note_off + sizeof(Elf32_Nhdr) + ALIGN(note_name_sz, 4);
 			memcpy(build_id, data, desc_sz);
 			memset(build_id + desc_sz, 0, BUILD_ID_SIZE_MAX - desc_sz);
 			if (size)
-- 
2.47.0