kdb: Censor attempts to set PROMPT without ENABLE_MEM_READ

Message ID 20200213164146.366251-1-daniel.thompson@linaro.org
State Accepted
Commit ad99b5105c0823ff02126497f4366e6a8009453e
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Series
  • kdb: Censor attempts to set PROMPT without ENABLE_MEM_READ
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Commit Message

Daniel Thompson Feb. 13, 2020, 4:41 p.m.
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(-)


base-commit: bb6d3fb354c5ee8d6bde2d576eb7220ea09862b9
--
2.23.0

Comments

Doug Anderson March 3, 2020, 8:51 p.m. | #1
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>

Patch

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);