Re: [PATCH v10 7/8] powerpc/bpf: fix buffer overflow in JIT for large BPF programs

On 05/08/26 11:37 am, Saket Kumar Bhaskar wrote:
> From: Abhishek Dubey <[email protected]>
> 
> During size calculation in pass-0, exit_addr is 0 since addrs[fp->len]
> is not yet populated. bpf_jit_emit_exit_insn() treats a zero exit_addr
> as in-range and skips bpf_jit_build_epilogue(), so the alternate inline
> epilogue instructions are not counted in alloclen.
> 
> In later passes, if the real exit_addr falls outside the 32MB branch
> range, the full inline epilogue is emitted into the already-allocated
> buffer, writing past its end and corrupting adjacent memory.
> 
> Fix by ensuring exit_addr is non-zero before treating it as in-range,
> so pass-0 always falls through to bpf_jit_build_epilogue() and
> conservatively accounts for all epilogue instructions in alloclen.
> Also range check alt_exit_addr directly in the else-if condition.
> 
> Since exit_addr handling now falls through to the epilogue, two
> related issues in bpf_int_jit_compile() must also be addressed:
> 
> 1. Reset cgctx.alt_exit_addr before the second size-calculation pass.
>     Without this, a stale alt_exit_addr from the first pass causes the
>     second pass to emit a single jump instead of the full epilogue,
>     undercounting alloclen and reintroducing the overflow.
> 
> 2. Recompute addrs[fp->len] at the end of each code-generation pass.
>     The larger pass-0 body can shrink in later passes as out-of-range
>     exits settle into in-range jumps; a stale addrs[fp->len] would
>     leave exit branches targeting past the real (shrunken) epilogue.
> 
> Because shrinkage in a later pass can move the epilogue offset, the
> fixed two-pass loop is no longer sufficient: an exit that was out of
> range in an earlier pass may fall in range once the epilogue offset
> shrinks, shrinking the body further and overwriting the start of the
> epilogue. Convert the code-generation loop to iterate until the
> program size converges, bounded by CODEGEN_MAX_PASSES, and fail the
> JIT if it does not converge.
> 
> Reported-by: [email protected]
> Closes: https://lore.kernel.org/bpf/[email protected]/T/#mfcb23909d977b949727cca4f59ee56a13fd69b92
> Fixes: d243b62b7bd3 ("powerpc64/bpf: Add support for bpf trampolines")
> Cc: [email protected]
> Signed-off-by: Hari Bathini <[email protected]>
> Signed-off-by: Abhishek Dubey <[email protected]>
> Signed-off-by: Saket Kumar Bhaskar <[email protected]>
> Link: https://lore.kernel.org/bpf/[email protected]/T/#mfcb23909d977b949727cca4f59ee56a13fd69b92
> Tested-by: Yeswanth Krishna Tellakula <[email protected]>
> ---
>   arch/powerpc/net/bpf_jit.h      |  7 +++++++
>   arch/powerpc/net/bpf_jit_comp.c | 34 +++++++++++++++++++++++++--------
>   2 files changed, 33 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index af510da12d8e..4da8bde92e1e 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -14,6 +14,13 @@
>   #include <asm/ppc-opcode.h>
>   #include <linux/build_bug.h>
>   
> +/*
> + * We need at least 2 passes for proper code generation, and may need
> + * additional passes if code size changes between passes.
> + */
> +#define CODEGEN_MIN_PASSES    2
> +#define CODEGEN_MAX_PASSES    3
> +
>   #ifdef CONFIG_PPC64_ELF_ABI_V1
>   #define FUNCTION_DESCR_SIZE	24
>   #else
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 8be5ded13a4a..3c20bb13cfd7 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -128,11 +128,10 @@ void bpf_jit_build_fentry_stubs(u32 *image, u32 *fimage, struct codegen_context
>   int bpf_jit_emit_exit_insn(u32 *image, u32 *fimage, struct codegen_context *ctx,
>   							int tmp_reg, long exit_addr)
>   {
> -	if (!exit_addr || is_offset_in_branch_range(exit_addr - (ctx->idx * 4))) {
> +	if (exit_addr && is_offset_in_branch_range(exit_addr - (long)(ctx->idx * 4))) {
>   		PPC_JMP(exit_addr);
> -	} else if (ctx->alt_exit_addr) {
> -		if (WARN_ON(!is_offset_in_branch_range((long)ctx->alt_exit_addr - (ctx->idx * 4))))
> -			return -1;
> +	} else if (ctx->alt_exit_addr && is_offset_in_branch_range(
> +			(long)(ctx->alt_exit_addr) - (long)(ctx->idx * 4))) {
>   		PPC_JMP(ctx->alt_exit_addr);
>   	} else {
>   		ctx->alt_exit_addr = ctx->idx * 4;
> @@ -303,6 +302,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
>   	 */
>   	if (cgctx.seen & SEEN_TAILCALL || !is_offset_in_branch_range((long)cgctx.idx * 4)) {
>   		cgctx.idx = 0;
> +		cgctx.alt_exit_addr = 0;
>   		if (bpf_jit_build_body(fp, NULL, NULL, &cgctx, addrs, 0, false))
>   			goto out_err;
>   	}
> @@ -335,10 +335,13 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
>   	code_base = (u32 *)(image + FUNCTION_DESCR_SIZE);
>   	fcode_base = (u32 *)(fimage + FUNCTION_DESCR_SIZE);
>   
> -	/* Code generation passes 1-2 */
> -	for (pass = 1; pass < 3; pass++) {
> +	/* Code generation passes 1-2+, loop until program size converges. */
> +	for (pass = 1; pass <= CODEGEN_MAX_PASSES; pass++) {
> +		u32 prev_proglen = proglen;
> +
>   		/* Now build the prologue, body code & epilogue for real. */
>   		cgctx.idx = 0;
> +		cgctx.exentry_idx = 0;
>   		cgctx.alt_exit_addr = 0;
>   		bpf_jit_build_prologue(code_base, &cgctx);
>   		if (bpf_jit_build_body(fp, code_base, fcode_base, &cgctx, addrs, pass,
> @@ -347,11 +350,26 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_verifier_env *env, struct bpf_pr
>   			bpf_jit_binary_pack_free(fhdr, hdr);
>   			goto out_err;
>   		}
> +		addrs[fp->len] = cgctx.idx * 4;
>   		bpf_jit_build_epilogue(code_base, fcode_base, &cgctx);
>   
> +		proglen = cgctx.idx * 4;
> +
>   		if (bpf_jit_enable > 1)
>   			pr_info("Pass %d: shrink = %d, seen = 0x%x\n", pass,
> -				proglen - (cgctx.idx * 4), cgctx.seen);
> +				prev_proglen - proglen, cgctx.seen);
> +
> +		/* Check if program size has converged, but ensure minimum passes */
> +		if (pass >= CODEGEN_MIN_PASSES && proglen == prev_proglen)
> +			break;
> +
> +		if (pass == CODEGEN_MAX_PASSES && proglen != prev_proglen) {
> +			pr_err("BPF JIT: Program did not converge after %d passes\n",
> +								CODEGEN_MAX_PASSES);
> +			bpf_arch_text_copy(&fhdr->size, &hdr->size, sizeof(hdr->size));
> +			bpf_jit_binary_pack_free(fhdr, hdr);
> +			goto out_err;
> +		}
>   	}
>   
>   	if (bpf_jit_enable > 1)
> @@ -428,7 +446,7 @@ int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, u32 *fimage, int pass
>   	u32 *fixup;
>   

>   	/* Populate extable entries only in the last pass */
> -	if (pass != 2)
> +	if (pass >= CODEGEN_MIN_PASSES)

This is wrong. Has to be:

     if (pass < CODEGEN_MIN_PASSES)


>   		return 0;
>   
>   	if (!fp->aux->extable ||

- Hari