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Message-ID: <20230824155234.GA102055@hpe.com>
Date: Thu, 24 Aug 2023 10:57:38 -0500
From: Dimitri Sivanich <sivanich@....com>
To: Kees Cook <keescook@...omium.org>
Cc: Justin Stitt <justinstitt@...gle.com>,
Andy Shevchenko <andy.shevchenko@...il.com>,
Steve Wahl <steve.wahl@....com>,
Dimitri Sivanich <dimitri.sivanich@....com>,
Russ Anderson <russ.anderson@....com>,
Darren Hart <dvhart@...radead.org>,
Andy Shevchenko <andy@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-hardening@...r.kernel.org
Subject: Re: [PATCH] x86/platform/uv: refactor deprecated strcpy and strncpy
On Wed, Aug 23, 2023 at 04:00:13PM -0700, Kees Cook wrote:
> On Wed, Aug 23, 2023 at 03:49:34PM -0700, Justin Stitt wrote:
> > On Wed, Aug 23, 2023 at 4:07 AM Andy Shevchenko
> > <andy.shevchenko@...il.com> wrote:
> > >
> > > On Wed, Aug 23, 2023 at 1:32 AM Justin Stitt <justinstitt@...gle.com> wrote:
> > > >
> > > > Both `strncpy` and `strcpy` are deprecated for use on NUL-terminated
> > > > destination strings [1].
> > > >
> > > > A suitable replacement is `strscpy` [2] due to the fact that it
> > > > guarantees NUL-termination on its destination buffer argument which is
> > > > _not_ the case for `strncpy` or `strcpy`!
> > > >
> > > > In this case, we can drop both the forced NUL-termination and the `... -1` from:
> > > > | strncpy(arg, val, ACTION_LEN - 1);
> > > > as `strscpy` implicitly has this behavior.
> > >
> > > ...
> > >
> > > > char arg[ACTION_LEN], *p;
> > > >
> > > > /* (remove possible '\n') */
> > > > - strncpy(arg, val, ACTION_LEN - 1);
> > > > - arg[ACTION_LEN - 1] = '\0';
> > > > + strscpy(arg, val, ACTION_LEN);
> > > > p = strchr(arg, '\n');
> > > > if (p)
> > > > *p = '\0';
> > >
> > > https://lore.kernel.org/all/202212091545310085328@zte.com.cn/
> > >
> > > ...
> > >
> > > > + strscpy(uv_nmi_action, arg, strlen(uv_nmi_action));
> > >
> > > strlen() on the destination?!
The original code for the above (strcpy()), copies strlen(arg) assuming null
termination, so strlen(uv_nmi_action) is not correct for this case. You
probably want to use sizeof of the destination.
> > >
> > > ...
> > >
> > > > - strncpy(uv_nmi_action, "dump", strlen(uv_nmi_action));
> > > > + strscpy(uv_nmi_action, "dump", strlen(uv_nmi_action));
> > >
> > > Again, this is weird.
> >
> > This is a common pattern with `strxcpy` and `sizeof` if you `$ rg
> > "strncpy\(.*sizeof"`. Do you recommend I switch the strlen(dest) to
> > strlen(src)? I only kept as-is because that's what was there
> > originally and I assumed some greater purpose of it.
>
> It's best to avoid any assumptions. If it can't be answered through code
> inspection, the next best thing would be to ask for clarification. In
> looking I see uv_nmi_action is a string:
>
> arch/x86/platform/uv/uv_nmi.c:193:typedef char action_t[ACTION_LEN];
>
> arch/x86/platform/uv/uv_nmi.c: strcpy(uv_nmi_action, arg);
> arch/x86/platform/uv/uv_nmi.c:module_param_named(action, uv_nmi_action, action, 0644);
> arch/x86/platform/uv/uv_nmi.c: return (strncmp(uv_nmi_action, action, strlen(action)) == 0);
> arch/x86/platform/uv/uv_nmi.c: strncpy(uv_nmi_action, "dump", strlen(uv_nmi_action));
>
> using strlen() here seems "accidentally safe", as it's overwriting
> "kdump":
>
> if (uv_nmi_action_is("kdump")) {
> uv_nmi_kdump(cpu, master, regs);
>
> /* Unexpected return, revert action to "dump" */
> if (master)
> strncpy(uv_nmi_action, "dump", strlen(uv_nmi_action));
>
> anyway, a simple "sizeof" should be used AFAICT.
>
I agree.
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