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bpf: Do not allow btf_ctx_access with __int128 types 

To ensure btf_ctx_access() is safe the verifier checks that the BTF
arg type is an int, enum, or pointer. When the function does the
BTF arg lookup it uses the calculation 'arg = off / 8'  using the
fact that registers are 8B. This requires that the first arg is
in the first reg, the second in the second, and so on. However,
for __int128 the arg will consume two registers by default LLVM
implementation. So this will cause the arg layout assumed by the
'arg = off / 8' calculation to be incorrect.

Because __int128 is uncommon this patch applies the easiest fix and
will force int types to be sizeof(u64) or smaller so that they will
fit in a single register.

v2: remove unneeded parens per Andrii's feedback

Fixes: 9e15db6613 ("bpf: Implement accurate raw_tp context access via BTF")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159303723962.11287.13309537171132420717.stgit@john-Precision-5820-Tower
This commit is contained in:
John Fastabend 2020-06-24 15:20:39 -07:00 committed by Daniel Borkmann
parent bcc7f554cf
commit a9b59159d3
2 changed files with 7 additions and 2 deletions
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5
include/linux/btf.h
View File

@ -82,6 +82,11 @@ static inline bool btf_type_is_int(const struct btf_type *t)
return BTF_INFO_KIND(t->info) == BTF_KIND_INT; return BTF_INFO_KIND(t->info) == BTF_KIND_INT;
} }
static inline bool btf_type_is_small_int(const struct btf_type *t)
{
return btf_type_is_int(t) && t->size <= sizeof(u64);
}
static inline bool btf_type_is_enum(const struct btf_type *t) static inline bool btf_type_is_enum(const struct btf_type *t)
{ {
return BTF_INFO_KIND(t->info) == BTF_KIND_ENUM; return BTF_INFO_KIND(t->info) == BTF_KIND_ENUM;

4
kernel/bpf/btf.c
View File

@ -3746,7 +3746,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
return false; return false;
t = btf_type_skip_modifiers(btf, t->type, NULL); t = btf_type_skip_modifiers(btf, t->type, NULL);
if (!btf_type_is_int(t)) { if (!btf_type_is_small_int(t)) {
bpf_log(log, bpf_log(log,
"ret type %s not allowed for fmod_ret\n", "ret type %s not allowed for fmod_ret\n",
btf_kind_str[BTF_INFO_KIND(t->info)]); btf_kind_str[BTF_INFO_KIND(t->info)]);
@ -3768,7 +3768,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
/* skip modifiers */ /* skip modifiers */
while (btf_type_is_modifier(t)) while (btf_type_is_modifier(t))
t = btf_type_by_id(btf, t->type); t = btf_type_by_id(btf, t->type);
if (btf_type_is_int(t) || btf_type_is_enum(t)) if (btf_type_is_small_int(t) || btf_type_is_enum(t))
/* accessing a scalar */ /* accessing a scalar */
return true; return true;
if (!btf_type_is_ptr(t)) { if (!btf_type_is_ptr(t)) {