Re: FAILED: patch "[PATCH] x86/uprobes: Do not use prefixes.nbytes when looping over" failed to apply to 4.9-stable tree

[Sudip Mukherjee <sudipm.mukherjee@gmail.com>]

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Hi Greg,

On Wed, Dec 09, 2020 at 10:34:11AM +0100, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 4.9-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

Here is the backport.

--
Regards
Sudip

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From 7437b190bde2ba9d0190eb25e11d1a549e45cfb9 Mon Sep 17 00:00:00 2001
From: Masami Hiramatsu <mhiramat@kernel.org>
Date: Thu, 3 Dec 2020 13:50:37 +0900
Subject: [PATCH] x86/uprobes: Do not use prefixes.nbytes when looping over prefixes.bytes

commit 4e9a5ae8df5b3365183150f6df49e49dece80d8c upstream

Since insn.prefixes.nbytes can be bigger than the size of
insn.prefixes.bytes[] when a prefix is repeated, the proper check must
be

  insn.prefixes.bytes[i] != 0 and i < 4

instead of using insn.prefixes.nbytes.

Introduce a for_each_insn_prefix() macro for this purpose. Debugged by
Kees Cook <keescook@chromium.org>.

 [ bp: Massage commit message, sync with the respective header in tools/
   and drop "we". ]

Fixes: 2b1444983508 ("uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints")
Reported-by: syzbot+9b64b619f10f19d19a7c@syzkaller.appspotmail.com
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/160697103739.3146288.7437620795200799020.stgit@devnote2
[sudip: adjust context, use old insn.h]
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
---
 arch/x86/include/asm/insn.h               | 15 +++++++++++++++
 arch/x86/kernel/uprobes.c                 | 10 ++++++----
 tools/objtool/arch/x86/include/asm/insn.h | 15 +++++++++++++++
 3 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/insn.h b/arch/x86/include/asm/insn.h
index c2c01f84df75..3e0e18d376d2 100644
--- a/arch/x86/include/asm/insn.h
+++ b/arch/x86/include/asm/insn.h
@@ -208,6 +208,21 @@ static inline int insn_offset_immediate(struct insn *insn)
 	return insn_offset_displacement(insn) + insn->displacement.nbytes;
 }
 
+/**
+ * for_each_insn_prefix() -- Iterate prefixes in the instruction
+ * @insn: Pointer to struct insn.
+ * @idx:  Index storage.
+ * @prefix: Prefix byte.
+ *
+ * Iterate prefix bytes of given @insn. Each prefix byte is stored in @prefix
+ * and the index is stored in @idx (note that this @idx is just for a cursor,
+ * do not change it.)
+ * Since prefixes.nbytes can be bigger than 4 if some prefixes
+ * are repeated, it cannot be used for looping over the prefixes.
+ */
+#define for_each_insn_prefix(insn, idx, prefix)	\
+	for (idx = 0; idx < ARRAY_SIZE(insn->prefixes.bytes) && (prefix = insn->prefixes.bytes[idx]) != 0; idx++)
+
 #define POP_SS_OPCODE 0x1f
 #define MOV_SREG_OPCODE 0x8e
 
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 73391c1bd2a9..52bb7413f352 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -268,10 +268,11 @@ static volatile u32 good_2byte_insns[256 / 32] = {
 
 static bool is_prefix_bad(struct insn *insn)
 {
+	insn_byte_t p;
 	int i;
 
-	for (i = 0; i < insn->prefixes.nbytes; i++) {
-		switch (insn->prefixes.bytes[i]) {
+	for_each_insn_prefix(insn, i, p) {
+		switch (p) {
 		case 0x26:	/* INAT_PFX_ES   */
 		case 0x2E:	/* INAT_PFX_CS   */
 		case 0x36:	/* INAT_PFX_DS   */
@@ -711,6 +712,7 @@ static const struct uprobe_xol_ops branch_xol_ops = {
 static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 {
 	u8 opc1 = OPCODE1(insn);
+	insn_byte_t p;
 	int i;
 
 	switch (opc1) {
@@ -741,8 +743,8 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 	 * Intel and AMD behavior differ in 64-bit mode: Intel ignores 66 prefix.
 	 * No one uses these insns, reject any branch insns with such prefix.
 	 */
-	for (i = 0; i < insn->prefixes.nbytes; i++) {
-		if (insn->prefixes.bytes[i] == 0x66)
+	for_each_insn_prefix(insn, i, p) {
+		if (p == 0x66)
 			return -ENOTSUPP;
 	}
 
diff --git a/tools/objtool/arch/x86/include/asm/insn.h b/tools/objtool/arch/x86/include/asm/insn.h
index b3e32b010ab1..b56241a44639 100644
--- a/tools/objtool/arch/x86/include/asm/insn.h
+++ b/tools/objtool/arch/x86/include/asm/insn.h
@@ -208,4 +208,19 @@ static inline int insn_offset_immediate(struct insn *insn)
 	return insn_offset_displacement(insn) + insn->displacement.nbytes;
 }
 
+/**
+ * for_each_insn_prefix() -- Iterate prefixes in the instruction
+ * @insn: Pointer to struct insn.
+ * @idx:  Index storage.
+ * @prefix: Prefix byte.
+ *
+ * Iterate prefix bytes of given @insn. Each prefix byte is stored in @prefix
+ * and the index is stored in @idx (note that this @idx is just for a cursor,
+ * do not change it.)
+ * Since prefixes.nbytes can be bigger than 4 if some prefixes
+ * are repeated, it cannot be used for looping over the prefixes.
+ */
+#define for_each_insn_prefix(insn, idx, prefix)        \
+	for (idx = 0; idx < ARRAY_SIZE(insn->prefixes.bytes) && (prefix = insn->prefixes.bytes[idx]) != 0; idx++)
+
 #endif /* _ASM_X86_INSN_H */
-- 
2.11.0