[PATCH v2 07/13] tracing/probes: Fix user-space dereference string array store type

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

When finalizing a string array argument (e.g. +0u(%reg):string[N]),
finalize_fetch_insn() advances the instruction pointer to a new slot
for the store operation before checking whether the preceding operation
was FETCH_OP_UDEREF. As a result, user-space dereference string arrays
with type "string" emit FETCH_OP_ST_STRING (kernel memory read) instead
of FETCH_OP_ST_USTRING.

Check for FETCH_OP_UDEREF before advancing the instruction pointer.

Fixes: e65f7ae7f4da ("tracing/probe: Support user-space dereference")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 kernel/trace/trace_probe.c |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
index 232d1deecb20..e98113808257 100644
--- a/kernel/trace/trace_probe.c
+++ b/kernel/trace/trace_probe.c
@@ -1953,6 +1953,8 @@ static int finalize_fetch_insn(struct fetch_insn *code,
 
 	/* Store operation */
 	if (parg->type->is_string) {
+		bool is_uderef;
+
 		/* Check bad combination of the type and the last fetch_insn. */
 		if (!strcmp(parg->type->name, "symstr")) {
 			if (code->op != FETCH_OP_REG && code->op != FETCH_OP_STACK &&
@@ -1972,6 +1974,7 @@ static int finalize_fetch_insn(struct fetch_insn *code,
 			}
 		}
 
+		is_uderef = (code->op == FETCH_OP_UDEREF);
 		if (!strcmp(parg->type->name, "symstr") ||
 		    (code->op == FETCH_OP_IMM || code->op == FETCH_OP_COMM ||
 		     code->op == FETCH_OP_IMMSTR) || code->op == FETCH_OP_TP_ARG ||
@@ -1992,8 +1995,7 @@ static int finalize_fetch_insn(struct fetch_insn *code,
 		}
 
 		/* If op == DEREF, replace it with STRING */
-		if (!strcmp(parg->type->name, "ustring") ||
-		    code->op == FETCH_OP_UDEREF)
+		if (!strcmp(parg->type->name, "ustring") || is_uderef)
 			code->op = FETCH_OP_ST_USTRING;
 		else if (!strcmp(parg->type->name, "symstr"))
 			code->op = FETCH_OP_ST_SYMSTR;