[PATCH] kdb: Censor attempts to set PROMPT without ENABLE_MEM_READ

[PATCH] kdb: Censor attempts to set PROMPT without ENABLE_MEM_READ

[Daniel Thompson]

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>
---
 kernel/debug/kdb/kdb_main.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index ba12e9f4661e..8dae08792641 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -398,6 +398,13 @@ 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
 	 */
@@ -1298,12 +1305,9 @@ 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);


base-commit: bb6d3fb354c5ee8d6bde2d576eb7220ea09862b9
--
2.23.0

Re: [PATCH] kdb: Censor attempts to set PROMPT without ENABLE_MEM_READ

[Doug Anderson]

Hi,

On Thu, Feb 13, 2020 at 8:42 AM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
>
> 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>
> ---
>  kernel/debug/kdb/kdb_main.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)

I can't say I'm an expert on the kdb permissions model since I wasn't
really even aware of it before reading this patch, but your change
seems reasonable to me.

Reviewed-by: Douglas Anderson <dianders@chromium.org>