diff mbox series

[bpf-next,v2,2/2] bpf, x86: Skip bounds checking for PROBE_MEM with SMAP

Message ID 20240619092216.1780946-3-memxor@gmail.com (mailing list archive)
State New, archived
Delegated to: BPF
Headers show
Series Zero overhead PROBE_MEM | expand

Checks

Context Check Description
netdev/series_format success Posting correctly formatted
netdev/tree_selection success Clearly marked for bpf-next
netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag not required for -next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit success Errors and warnings before: 8 this patch: 8
netdev/build_tools success No tools touched, skip
netdev/cc_maintainers warning 8 maintainers not CCed: jolsa@kernel.org john.fastabend@gmail.com haoluo@google.com martin.lau@linux.dev netdev@vger.kernel.org dsahern@kernel.org kpsingh@kernel.org sdf@google.com
netdev/build_clang success Errors and warnings before: 854 this patch: 854
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
netdev/build_allmodconfig_warn success Errors and warnings before: 854 this patch: 854
netdev/checkpatch warning WARNING: labels should not be indented WARNING: line length of 114 exceeds 80 columns
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
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bpf/vmtest-bpf-next-VM_Test-42 success Logs for x86_64-llvm-18 / veristat
bpf/vmtest-bpf-next-VM_Test-6 success Logs for aarch64-gcc / test (test_maps, false, 360) / test_maps on aarch64 with gcc
bpf/vmtest-bpf-next-VM_Test-9 success Logs for aarch64-gcc / test (test_verifier, false, 360) / test_verifier on aarch64 with gcc
bpf/vmtest-bpf-next-VM_Test-21 success Logs for x86_64-gcc / test (test_maps, false, 360) / test_maps on x86_64 with gcc
bpf/vmtest-bpf-next-VM_Test-22 success Logs for x86_64-gcc / test (test_progs, false, 360) / test_progs on x86_64 with gcc
bpf/vmtest-bpf-next-VM_Test-23 success Logs for x86_64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with gcc
bpf/vmtest-bpf-next-VM_Test-24 success Logs for x86_64-gcc / test (test_progs_no_alu32_parallel, true, 30) / test_progs_no_alu32_parallel on x86_64 with gcc
bpf/vmtest-bpf-next-VM_Test-26 success Logs for x86_64-gcc / test (test_verifier, false, 360) / test_verifier on x86_64 with gcc
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bpf/vmtest-bpf-next-VM_Test-30 success Logs for x86_64-llvm-17 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-17
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bpf/vmtest-bpf-next-VM_Test-40 success Logs for x86_64-llvm-18 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-18
bpf/vmtest-bpf-next-VM_Test-8 success Logs for aarch64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on aarch64 with gcc
bpf/vmtest-bpf-next-VM_Test-7 success Logs for aarch64-gcc / test (test_progs, false, 360) / test_progs on aarch64 with gcc
bpf/vmtest-bpf-next-VM_Test-16 success Logs for s390x-gcc / test (test_verifier, false, 360) / test_verifier on s390x with gcc
bpf/vmtest-bpf-next-VM_Test-13 success Logs for s390x-gcc / test (test_maps, false, 360) / test_maps on s390x with gcc
bpf/vmtest-bpf-next-VM_Test-15 success Logs for s390x-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on s390x with gcc
bpf/vmtest-bpf-next-PR fail PR summary
bpf/vmtest-bpf-next-VM_Test-14 success Logs for s390x-gcc / test (test_progs, false, 360) / test_progs on s390x with gcc

Commit Message

Kumar Kartikeya Dwivedi June 19, 2024, 9:22 a.m. UTC 
The previous patch changed the do_user_addr_fault page fault handler to
invoke BPF's fixup routines (by searching exception tables and calling
ex_handler_bpf). This would only occur when SMAP is enabled, such that
any user address access from BPF programs running in kernel mode would
reach this path and invoke the fixup routines.

Relying on this behavior, disable any bounds checking instrumentation in
the BPF JIT for x86 when X86_FEATURE_SMAP is available. All BPF
programs execute with SMAP enabled, therefore when this feature is
available, we can assume that SMAP will be enabled during program
execution at runtime.

This optimizes PROBE_MEM loads down to a normal unchecked load
instruction. Any page faults for user or kernel addresses will be
handled using the fixup routines, and the generation exception table
entries for such load instructions.

All in all, this ensures that PROBE_MEM loads will now incur no runtime
overhead, and become practically free.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 arch/x86/net/bpf_jit_comp.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)
diff mbox series

Patch

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 5159c7a22922..f8a39189cddc 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1864,8 +1864,8 @@  st:			if (is_imm8(insn->off))
 		case BPF_LDX | BPF_PROBE_MEMSX | BPF_W:
 			insn_off = insn->off;
 
-			if (BPF_MODE(insn->code) == BPF_PROBE_MEM ||
-			    BPF_MODE(insn->code) == BPF_PROBE_MEMSX) {
+			if ((BPF_MODE(insn->code) == BPF_PROBE_MEM ||
+			     BPF_MODE(insn->code) == BPF_PROBE_MEMSX) && !cpu_feature_enabled(X86_FEATURE_SMAP)) {
 				/* Conservatively check that src_reg + insn->off is a kernel address:
 				 *   src_reg + insn->off > TASK_SIZE_MAX + PAGE_SIZE
 				 *   and
@@ -1912,6 +1912,9 @@  st:			if (is_imm8(insn->off))
 				/* populate jmp_offset for JAE above to jump to start_of_ldx */
 				start_of_ldx = prog;
 				end_of_jmp[-1] = start_of_ldx - end_of_jmp;
+			} else if ((BPF_MODE(insn->code) == BPF_PROBE_MEM ||
+				    BPF_MODE(insn->code) == BPF_PROBE_MEMSX)) {
+				start_of_ldx = prog;
 			}
 			if (BPF_MODE(insn->code) == BPF_PROBE_MEMSX ||
 			    BPF_MODE(insn->code) == BPF_MEMSX)
@@ -1924,9 +1927,13 @@  st:			if (is_imm8(insn->off))
 				u8 *_insn = image + proglen + (start_of_ldx - temp);
 				s64 delta;
 
+				if (cpu_feature_enabled(X86_FEATURE_SMAP))
+					goto extable_fixup;
+
 				/* populate jmp_offset for JMP above */
 				start_of_ldx[-1] = prog - start_of_ldx;
 
+			extable_fixup:
 				if (!bpf_prog->aux->extable)
 					break;