2020-02-13 16:43:33

by Daniel Thompson

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

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 <[email protected]>
Signed-off-by: Daniel Thompson <[email protected]>
---
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

2020-03-03 21:48:51

by Doug Anderson

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

Hi,

On Thu, Feb 13, 2020 at 8:42 AM Daniel Thompson
<[email protected]> 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 <[email protected]>
> Signed-off-by: Daniel Thompson <[email protected]>
> ---
> 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 <[email protected]>