Messages in this thread Patch in this message | | | From | "Hook, Gary" <> | Subject | [PATCH] x86/mm/mem_encrypt: Disable all instrumentation for SME early boot code | Date | Thu, 4 Apr 2019 20:26:55 +0000 |
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Enablement of AMD's Secure Memory Encryption feature is determined very early in the boot cycle. Part of this procedure involves scanning the command line for the paramater 'mem_encrypt'.
To determine intended state, the function sme_enable() uses library functions cmdline_find_option() and strncmp(). Their use occurs early enough such that we can't assume that any instrumentation subsystem is initialized. For example, making calls to a KASAN-instrumented function before KASAN is set up will likely result in the use of uninitialized memory and a boot failure.
Avoid instrumenting these dependent functions by:
1) Making a local, static, renamed copy of strncpy() for use solely in mem_encrypt_identity.c. In this file we are able to vet its few uses and avoid exposing the rest of the kernel to a ubiquitously used but un-instrumented function.
2) Disable instrumention of arch/x86/lib/cmdline.c based on the assumption that the needed function (cmdline_find_option()) is vetted through its use to date, and contains no lurking flaws that have not yet been found through instrumentation such as KASAN.
Fixes: aca20d546214 ("x86/mm: Add support to make use of Secure Memory Encryption") Reported-by: Li RongQing <lirongqing@baidu.com> Signed-off-by: Gary R Hook <gary.hook@amd.com> --- arch/x86/lib/Makefile | 13 +++++++++++++ arch/x86/mm/mem_encrypt_identity.c | 26 ++++++++++++++++++++++++-- 2 files changed, 37 insertions(+), 2 deletions(-)
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile index 140e61843a07..38182a64fa81 100644 --- a/arch/x86/lib/Makefile +++ b/arch/x86/lib/Makefile @@ -6,6 +6,19 @@ # Produces uninteresting flaky coverage. KCOV_INSTRUMENT_delay.o := n +# SME early boot code checks the cmdline, so don't instrument +KCOV_INSTRUMENT_cmdline.o := n + +KASAN_SANITIZE_cmdline.o := n + +ifdef CONFIG_FUNCTION_TRACER +CFLAGS_REMOVE_cmdline.o = -pg +endif + +# No stack protector +nostackp := $(call cc-option, -fno-stack-protector) +CFLAGS_cmdline.o := $(nostackp) + inat_tables_script = $(srctree)/arch/x86/tools/gen-insn-attr-x86.awk inat_tables_maps = $(srctree)/arch/x86/lib/x86-opcode-map.txt quiet_cmd_inat_tables = GEN $@ diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c index 4aa9b1480866..0a68d7ccb371 100644 --- a/arch/x86/mm/mem_encrypt_identity.c +++ b/arch/x86/mm/mem_encrypt_identity.c @@ -77,6 +77,28 @@ static char sme_cmdline_arg[] __initdata = "mem_encrypt"; static char sme_cmdline_on[] __initdata = "on"; static char sme_cmdline_off[] __initdata = "off"; +/* + * Local copy to avoid instrumentation + * Copied from lib/string.c and renamed to be unique. + * This file is early boot code, and we assume that instrumentation + * subsystems (e.g. KASAN) are not yet initialized. + */ +static int sme_strncmp(const char *cs, const char *ct, size_t count) +{ + unsigned char c1, c2; + + while (count) { + c1 = *cs++; + c2 = *ct++; + if (c1 != c2) + return c1 < c2 ? -1 : 1; + if (!c1) + break; + count--; + } + return 0; +} + static void __init sme_clear_pgd(struct sme_populate_pgd_data *ppd) { unsigned long pgd_start, pgd_end, pgd_size; @@ -557,9 +579,9 @@ void __init sme_enable(struct boot_params *bp) cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer)); - if (!strncmp(buffer, cmdline_on, sizeof(buffer))) + if (!sme_strncmp(buffer, cmdline_on, sizeof(buffer))) sme_me_mask = me_mask; - else if (!strncmp(buffer, cmdline_off, sizeof(buffer))) + else if (!sme_strncmp(buffer, cmdline_off, sizeof(buffer))) sme_me_mask = 0; else sme_me_mask = active_by_default ? me_mask : 0;
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