From: Will Deacon will@kernel.org
commit 0613d8ca9ab382caabe9ed2dceb429e9781e443f upstream.
A narrow load from a 64-bit context field results in a 64-bit load followed potentially by a 64-bit right-shift and then a bitwise AND operation to extract the relevant data.
In the case of a 32-bit access, an immediate mask of 0xffffffff is used to construct a 64-bit BPP_AND operation which then sign-extends the mask value and effectively acts as a glorified no-op. For example:
0: 61 10 00 00 00 00 00 00 r0 = *(u32 *)(r1 + 0)
results in the following code generation for a 64-bit field:
ldr x7, [x7] // 64-bit load mov x10, #0xffffffffffffffff and x7, x7, x10
Fix the mask generation so that narrow loads always perform a 32-bit AND operation:
ldr x7, [x7] // 64-bit load mov w10, #0xffffffff and w7, w7, w10
Cc: Alexei Starovoitov ast@kernel.org Cc: Daniel Borkmann daniel@iogearbox.net Cc: John Fastabend john.fastabend@gmail.com Cc: Krzesimir Nowak krzesimir@kinvolk.io Cc: Andrey Ignatov rdna@fb.com Acked-by: Yonghong Song yhs@fb.com Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields") Signed-off-by: Will Deacon will@kernel.org Link: https://lore.kernel.org/r/20230518102528.1341-1-will@kernel.org Signed-off-by: Alexei Starovoitov ast@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/bpf/verifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5936,7 +5936,7 @@ static int convert_ctx_accesses(struct b insn_buf[cnt++] = BPF_ALU64_IMM(BPF_RSH, insn->dst_reg, shift); - insn_buf[cnt++] = BPF_ALU64_IMM(BPF_AND, insn->dst_reg, + insn_buf[cnt++] = BPF_ALU32_IMM(BPF_AND, insn->dst_reg, (1ULL << size * 8) - 1); } }