From: Luke Nelson lukenels@cs.washington.edu
commit bb9562cf5c67813034c96afb50bd21130a504441 upstream.
The current arm BPF JIT does not correctly compile RSH or ARSH when the immediate shift amount is 0. This causes the "rsh64 by 0 imm" and "arsh64 by 0 imm" BPF selftests to hang the kernel by reaching an instruction the verifier determines to be unreachable.
The root cause is in how immediate right shifts are encoded on arm. For LSR and ASR (logical and arithmetic right shift), a bit-pattern of 00000 in the immediate encodes a shift amount of 32. When the BPF immediate is 0, the generated code shifts by 32 instead of the expected behavior (a no-op).
This patch fixes the bugs by adding an additional check if the BPF immediate is 0. After the change, the above mentioned BPF selftests pass.
Fixes: 39c13c204bb11 ("arm: eBPF JIT compiler") Co-developed-by: Xi Wang xi.wang@gmail.com Signed-off-by: Xi Wang xi.wang@gmail.com Signed-off-by: Luke Nelson luke.r.nels@gmail.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Link: https://lore.kernel.org/bpf/20200408181229.10909-1-luke.r.nels@gmail.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/arm/net/bpf_jit_32.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)
--- a/arch/arm/net/bpf_jit_32.c +++ b/arch/arm/net/bpf_jit_32.c @@ -930,7 +930,11 @@ static inline void emit_a32_rsh_i64(cons rd = arm_bpf_get_reg64(dst, tmp, ctx);
/* Do LSR operation */ - if (val < 32) { + if (val == 0) { + /* An immediate value of 0 encodes a shift amount of 32 + * for LSR. To shift by 0, don't do anything. + */ + } else if (val < 32) { emit(ARM_MOV_SI(tmp2[1], rd[1], SRTYPE_LSR, val), ctx); emit(ARM_ORR_SI(rd[1], tmp2[1], rd[0], SRTYPE_ASL, 32 - val), ctx); emit(ARM_MOV_SI(rd[0], rd[0], SRTYPE_LSR, val), ctx); @@ -956,7 +960,11 @@ static inline void emit_a32_arsh_i64(con rd = arm_bpf_get_reg64(dst, tmp, ctx);
/* Do ARSH operation */ - if (val < 32) { + if (val == 0) { + /* An immediate value of 0 encodes a shift amount of 32 + * for ASR. To shift by 0, don't do anything. + */ + } else if (val < 32) { emit(ARM_MOV_SI(tmp2[1], rd[1], SRTYPE_LSR, val), ctx); emit(ARM_ORR_SI(rd[1], tmp2[1], rd[0], SRTYPE_ASL, 32 - val), ctx); emit(ARM_MOV_SI(rd[0], rd[0], SRTYPE_ASR, val), ctx);