Re: [PATCH v5 12/12] x86/bugs: Add a separate config for missing mitigation

[Breno Leitao <leitao@debian.org>]

Hello Josh,

On Wed, Oct 25, 2023 at 09:29:06AM -0700, Josh Poimboeuf wrote:
> On Thu, Oct 19, 2023 at 11:11:58AM -0700, Breno Leitao wrote:
> > Currently, the CONFIG_SPECULATION_MITIGATIONS is halfway populated,
> > where some mitigations have entries in Kconfig, and they could be
> > modified, while others mitigations do not have Kconfig entries, and
> > could not be controlled at build time.
> > 
> > Create an entry for each CPU mitigation under
> > CONFIG_SPECULATION_MITIGATIONS. This allow users to enable or disable
> > them at compilation time.
> > 
> > Signed-off-by: Breno Leitao <leitao@debian.org>
> 
> We also probably need a CONFIG_MITIGATION_MELTDOWN.

Isn't Meltdown covered by the MITIGATION_PAGE_TABLE_ISOLATION Kconfig
entry? Would you mind clarifying what would be the difference between
CONFIG_MITIGATION_MELTDOWN and MITIGATION_PAGE_TABLE_ISOLATION, and why
do we want CONFIG_MITIGATION_MELTDOWN?

> > ---
> >  arch/x86/Kconfig           | 93 ++++++++++++++++++++++++++++++++++++++
> >  arch/x86/kernel/cpu/bugs.c | 39 ++++++++++------
> >  2 files changed, 117 insertions(+), 15 deletions(-)

<snip>

> > +config MITIGATION_SRBDS
> > +	bool "Mitigate Special Register Buffer Data Sampling (SRBDS) hardware bug"
> > +	depends on CPU_SUP_INTEL
> > +	default y
> > +	help
> > +	  Enable mitigation for Special Register Buffer Data Sampling (SRBDS).
> > +	  SRBDS is a hardware vulnerability that allows Microarchitectural Data
> > +	  Sampling (MDS) techniques to infer values returned from special
> > +	  register accesses. An unprivileged user can extract values returned
> > +	  from RDRAND and RDSEED executed on another core or sibling thread
> > +	  using MDS techniques.
> 
> Refer to Documentation/admin-guide/hw-vuln/special-register-buffer-data-sampling.rst

Sure, I will update this and all the other suggestions that were cut
above. Thanks!

> > +	cmd = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ?  SPECTRE_V2_CMD_AUTO : SPECTRE_V2_CMD_NONE;
> >  	if (cmdline_find_option_bool(boot_command_line, "nospectre_v2") ||
> >  	    cpu_mitigations_off())
> >  		return SPECTRE_V2_CMD_NONE;
> 
> I'm thinking CONFIG_MITIGATION_SPECTRE_V2 should also affect whether the spectre v2 user
> mitigation gets enabled.

Makes sense, would something like this be enough?

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 11ccbadd8800..cfcdbfa72a81 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1221,8 +1221,10 @@ static __ro_after_init enum spectre_v2_mitigation_cmd spectre_v2_cmd;
 static enum spectre_v2_user_cmd __init
 spectre_v2_parse_user_cmdline(void)
 {
+       int ret, i, mode;
        char arg[20];
-       int ret, i;
+
+       mode = IS_ENABLED(CONFIG_MITIGATION_SPECTRE_V2) ? SPECTRE_V2_USER_CMD_AUTO : SPECTRE_V2_USER_CMD_NONE;

        switch (spectre_v2_cmd) {
        case SPECTRE_V2_CMD_NONE:
@@ -1236,7 +1238,7 @@ spectre_v2_parse_user_cmdline(void)
        ret = cmdline_find_option(boot_command_line, "spectre_v2_user",
                                  arg, sizeof(arg));
        if (ret < 0)
-               return SPECTRE_V2_USER_CMD_AUTO;
+               return mode;

        for (i = 0; i < ARRAY_SIZE(v2_user_options); i++) {
                if (match_option(arg, ret, v2_user_options[i].option)) {
@@ -1246,8 +1248,8 @@ spectre_v2_parse_user_cmdline(void)
                }
        }

-       pr_err("Unknown user space protection option (%s). Switching to AUTO select\n", arg);
-       return SPECTRE_V2_USER_CMD_AUTO;
+       pr_err("Unknown user space protection option (%s). Switching to default\n", arg);
+       return mode;
 }