linux-kernel - [tip: x86/fpu] x86/fpu: Allow multiple bits in clearcpuid= parameter

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Date:   Thu, 10 Sep 2020 16:37:43 -0000
From:   "tip-bot2 for Arvind Sankar" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Borislav Petkov <bp@...e.de>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [tip: x86/fpu] x86/fpu: Allow multiple bits in clearcpuid= parameter

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     0a4bb5e5507a585532cc413125b921c8546fc39f
Gitweb:        https://git.kernel.org/tip/0a4bb5e5507a585532cc413125b921c8546fc39f
Author:        Arvind Sankar <nivedita@...m.mit.edu>
AuthorDate:    Mon, 07 Sep 2020 17:39:19 -04:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Thu, 10 Sep 2020 18:32:05 +02:00

x86/fpu: Allow multiple bits in clearcpuid= parameter

Commit

  0c2a3913d6f5 ("x86/fpu: Parse clearcpuid= as early XSAVE argument")

changed clearcpuid parsing from __setup() to cmdline_find_option().
While the __setup() function would have been called for each clearcpuid=
parameter on the command line, cmdline_find_option() will only return
the last one, so the change effectively made it impossible to disable
more than one bit.

Allow a comma-separated list of bit numbers as the argument for
clearcpuid to allow multiple bits to be disabled again. Log the bits
being disabled for informational purposes.

Also fix the check on the return value of cmdline_find_option(). It
returns -1 when the option is not found, so testing as a boolean is
incorrect.

Fixes: 0c2a3913d6f5 ("x86/fpu: Parse clearcpuid= as early XSAVE argument")
Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lkml.kernel.org/r/20200907213919.2423441-1-nivedita@alum.mit.edu
---
 Documentation/admin-guide/kernel-parameters.txt |  2 +-
 arch/x86/kernel/fpu/init.c                      | 30 +++++++++++-----
 2 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a106874..ffe8643 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -577,7 +577,7 @@
 			loops can be debugged more effectively on production
 			systems.
 
-	clearcpuid=BITNUM [X86]
+	clearcpuid=BITNUM[,BITNUM...] [X86]
 			Disable CPUID feature X for the kernel. See
 			arch/x86/include/asm/cpufeatures.h for the valid bit
 			numbers. Note the Linux specific bits are not necessarily
diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
index 61ddc3a..f8ff895 100644
--- a/arch/x86/kernel/fpu/init.c
+++ b/arch/x86/kernel/fpu/init.c
@@ -243,9 +243,9 @@ static void __init fpu__init_system_ctx_switch(void)
  */
 static void __init fpu__init_parse_early_param(void)
 {
-	char arg[32];
+	char arg[128];
 	char *argptr = arg;
-	int bit;
+	int arglen, res, bit;
 
 #ifdef CONFIG_X86_32
 	if (cmdline_find_option_bool(boot_command_line, "no387"))
@@ -268,12 +268,26 @@ static void __init fpu__init_parse_early_param(void)
 	if (cmdline_find_option_bool(boot_command_line, "noxsaves"))
 		setup_clear_cpu_cap(X86_FEATURE_XSAVES);
 
-	if (cmdline_find_option(boot_command_line, "clearcpuid", arg,
-				sizeof(arg)) &&
-	    get_option(&argptr, &bit) &&
-	    bit >= 0 &&
-	    bit < NCAPINTS * 32)
-		setup_clear_cpu_cap(bit);
+	arglen = cmdline_find_option(boot_command_line, "clearcpuid", arg, sizeof(arg));
+	if (arglen <= 0)
+		return;
+
+	pr_info("Clearing CPUID bits:");
+	do {
+		res = get_option(&argptr, &bit);
+		if (res == 0 || res == 3)
+			break;
+
+		/* If the argument was too long, the last bit may be cut off */
+		if (res == 1 && arglen >= sizeof(arg))
+			break;
+
+		if (bit >= 0 && bit < NCAPINTS * 32) {
+			pr_cont(" " X86_CAP_FMT, x86_cap_flag(bit));
+			setup_clear_cpu_cap(bit);
+		}
+	} while (res == 2);
+	pr_cont("\n");
 }
 
 /*