kdb: Censor attempts to set PROMPT without ENABLE_MEM_READ

[ Upstream commit ad99b5105c0823ff02126497f4366e6a8009453e ]

Currently the PROMPT variable could be abused to provoke the printf()
machinery to read outside the current stack frame. Normally this
doesn't matter becaues md is already a much better tool for reading
from memory.

However the md command can be disabled by not setting KDB_ENABLE_MEM_READ.
Let's also prevent PROMPT from being modified in these circumstances.

Whilst adding a comment to help future code reviewers we also remove
the #ifdef where PROMPT in consumed. There is no problem passing an
unused (0) to snprintf when !CONFIG_SMP.
argument

Reported-by: Wang Xiayang <xywang.sjtu@sjtu.edu.cn>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Stable-dep-of: 4f41d30cd6dc ("kdb: Fix a potential buffer overflow in kdb_local()")
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/debug/kdb/kdb_main.c

@@ -452,6 +452,13 @@ int kdb_set(int argc, const char **argv)
 	if (argc != 2)
 		return KDB_ARGCOUNT;
 
+	/*
+	 * Censor sensitive variables
+	 */
+	if (strcmp(argv[1], "PROMPT") == 0 &&
+	    !kdb_check_flags(KDB_ENABLE_MEM_READ, kdb_cmd_enabled, false))
+		return KDB_NOPERM;
+
 	/*
 	 * Check for internal variables
 	 */
@@ -1355,12 +1362,9 @@ static int kdb_local(kdb_reason_t reason, int error, struct pt_regs *regs,
 		*(cmd_hist[cmd_head]) = '\0';
 
 do_full_getstr:
-#if defined(CONFIG_SMP)
+		/* PROMPT can only be set if we have MEM_READ permission. */
 		snprintf(kdb_prompt_str, CMD_BUFLEN, kdbgetenv("PROMPT"),
 			 raw_smp_processor_id());
-#else
-		snprintf(kdb_prompt_str, CMD_BUFLEN, kdbgetenv("PROMPT"));
-#endif
 		if (defcmd_in_progress)
 			strncat(kdb_prompt_str, "[defcmd]", CMD_BUFLEN);