[PATCH AUTOSEL for 4.4 067/115] MIPS: BPF: Quit clobbering callee saved registers in JIT code.

[Sasha Levin]

From: David Daney <david.daney@xxxxxxxxxx>

[ 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.

Signed-off-by: David Daney <david.daney@xxxxxxxxxx>
Cc: James Hogan <james.hogan@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Steven J. Hill <steven.hill@xxxxxxxxxx>
Cc: linux-mips@xxxxxxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Patchwork: https://patchwork.linux-mips.org/patch/15745/
Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
---
 arch/mips/net/bpf_jit.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
index 1a8c96035716..c0c1e9529dbd 100644
--- a/arch/mips/net/bpf_jit.c
+++ b/arch/mips/net/bpf_jit.c
@@ -527,7 +527,8 @@ static void save_bpf_jit_regs(struct jit_ctx *ctx, unsigned offset)
 	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 jit_ctx *ctx,
 		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_ctx *ctx)
 	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);
 }
 
-- 
2.14.1