Patch "MIPS: BPF: Quit clobbering callee saved registers in JIT code." has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

MIPS: BPF: Quit clobbering callee saved registers in JIT code.

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mips-bpf-quit-clobbering-callee-saved-registers-in-jit-code.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: David Daney <[email protected]>
Date: Tue, 14 Mar 2017 14:21:43 -0700
Subject: MIPS: BPF: Quit clobbering callee saved registers in JIT code.

From: David Daney <[email protected]>

[ Upstream commit 1ef0910cfd681f0bd0b81f8809935b2006e9cfb9 ]

If bpf_needs_clear_a() returns true, only actually clear it if it is ever used. If it is not used, we don't save and restore it, so the clearing has the nasty side effect of clobbering caller state.

Also, don't emit stack pointer adjustment instructions if the adjustment amount is zero.

Patchwork: https://patchwork.linux-mips.org/patch/15745/

---
 arch/mips/net/bpf_jit.c |   16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

@@ -527,7 +527,8 @@ static void save_bpf_jit_regs(struct jit
 	u32 sflags, tmp_flags;
 
 	/* Adjust the stack pointer */
-	emit_stack_offset(-align_sp(offset), ctx);
+	if (offset)
+		emit_stack_offset(-align_sp(offset), ctx);
 
 	tmp_flags = sflags = ctx->flags >> SEEN_SREG_SFT;
 	/* sflags is essentially a bitmap */

@@ -579,7 +580,8 @@ static void restore_bpf_jit_regs(struct
 		emit_load_stack_reg(r_ra, r_sp, real_off, ctx);
 
 	/* Restore the sp and discard the scrach memory */
-	emit_stack_offset(align_sp(offset), ctx);
+	if (offset)
+		emit_stack_offset(align_sp(offset), ctx);
 }
 
 static unsigned int get_stack_depth(struct jit_ctx *ctx)

@@ -626,8 +628,14 @@ static void build_prologue(struct jit_ct
 	if (ctx->flags & SEEN_X)
 		emit_jit_reg_move(r_X, r_zero, ctx);
 
-	/* Do not leak kernel data to userspace */
-	if (bpf_needs_clear_a(&ctx->skf->insns[0]))
+	/*
+	 * Do not leak kernel data to userspace, we only need to clear
+	 * r_A if it is ever used.  In fact if it is never used, we
+	 * will not save/restore it, so clearing it in this case would
+	 * corrupt the state of the caller.
+	 */
+	if (bpf_needs_clear_a(&ctx->skf->insns[0]) &&
+	    (ctx->flags & SEEN_A))
 		emit_jit_reg_move(r_A, r_zero, ctx);
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/mips-bpf-quit-clobbering-callee-saved-registers-in-jit-code.patch queue-4.4/mips-bpf-fix-multiple-problems-in-jit-skb-access-helpers.patch