From 3aac3ebea08f2d342364f827c8979ab0e1dd591e Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Thu, 21 Oct 2021 15:55:06 -0700 Subject: [PATCH] x86/signal: Implement sigaltstack size validation For historical reasons MINSIGSTKSZ is a constant which became already too small with AVX512 support. Add a mechanism to enforce strict checking of the sigaltstack size against the real size of the FPU frame. The strict check can be enabled via a config option and can also be controlled via the kernel command line option 'strict_sas_size' independent of the config switch. Enabling it might break existing applications which allocate a too small sigaltstack but 'work' because they never get a signal delivered. Though it can be handy to filter out binaries which are not yet aware of AT_MINSIGSTKSZ. Also the upcoming support for dynamically enabled FPU features requires a strict sanity check to ensure that: - Enabling of a dynamic feature, which changes the sigframe size fits into an enabled sigaltstack - Installing a too small sigaltstack after a dynamic feature has been added is not possible. Implement the base check which is controlled by config and command line options. Signed-off-by: Thomas Gleixner Signed-off-by: Chang S. Bae Signed-off-by: Borislav Petkov Link: https://lkml.kernel.org/r/20211021225527.10184-3-chang.seok.bae@intel.com --- .../admin-guide/kernel-parameters.txt | 9 +++++ arch/x86/Kconfig | 17 +++++++++ arch/x86/kernel/signal.c | 35 +++++++++++++++++++ 3 files changed, 61 insertions(+) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index 43dc35fe5bc0..eb9a73ad91a0 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -5497,6 +5497,15 @@ stifb= [HW] Format: bpp:[:[:...]] + strict_sas_size= + [X86] + Format: + Enable or disable strict sigaltstack size checks + against the required signal frame size which + depends on the supported FPU features. This can + be used to filter out binaries which have + not yet been made aware of AT_MINSIGSTKSZ. + sunrpc.min_resvport= sunrpc.max_resvport= [NFS,SUNRPC] diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index d9830e7e1060..8584b30e2536 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -125,6 +125,7 @@ config X86 select CLOCKSOURCE_VALIDATE_LAST_CYCLE select CLOCKSOURCE_WATCHDOG select DCACHE_WORD_ACCESS + select DYNAMIC_SIGFRAME select EDAC_ATOMIC_SCRUB select EDAC_SUPPORT select GENERIC_CLOCKEVENTS_BROADCAST if X86_64 || (X86_32 && X86_LOCAL_APIC) @@ -2388,6 +2389,22 @@ config MODIFY_LDT_SYSCALL Saying 'N' here may make sense for embedded or server kernels. +config STRICT_SIGALTSTACK_SIZE + bool "Enforce strict size checking for sigaltstack" + depends on DYNAMIC_SIGFRAME + help + For historical reasons MINSIGSTKSZ is a constant which became + already too small with AVX512 support. Add a mechanism to + enforce strict checking of the sigaltstack size against the + real size of the FPU frame. This option enables the check + by default. It can also be controlled via the kernel command + line option 'strict_sas_size' independent of this config + switch. Enabling it might break existing applications which + allocate a too small sigaltstack but 'work' because they + never get a signal delivered. + + Say 'N' unless you want to really enforce this check. + source "kernel/livepatch/Kconfig" endmenu diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c index 58bd07071d14..0111a6ae6e60 100644 --- a/arch/x86/kernel/signal.c +++ b/arch/x86/kernel/signal.c @@ -15,6 +15,7 @@ #include #include #include +#include #include #include #include @@ -40,6 +41,7 @@ #include #include #include +#include #endif /* CONFIG_X86_64 */ #include @@ -907,6 +909,39 @@ void signal_fault(struct pt_regs *regs, void __user *frame, char *where) force_sig(SIGSEGV); } +#ifdef CONFIG_DYNAMIC_SIGFRAME +#ifdef CONFIG_STRICT_SIGALTSTACK_SIZE +static bool strict_sigaltstack_size __ro_after_init = true; +#else +static bool strict_sigaltstack_size __ro_after_init = false; +#endif + +static int __init strict_sas_size(char *arg) +{ + return kstrtobool(arg, &strict_sigaltstack_size); +} +__setup("strict_sas_size", strict_sas_size); + +/* + * MINSIGSTKSZ is 2048 and can't be changed despite the fact that AVX512 + * exceeds that size already. As such programs might never use the + * sigaltstack they just continued to work. While always checking against + * the real size would be correct, this might be considered a regression. + * + * Therefore avoid the sanity check, unless enforced by kernel config or + * command line option. + */ +bool sigaltstack_size_valid(size_t ss_size) +{ + lockdep_assert_held(¤t->sighand->siglock); + + if (strict_sigaltstack_size) + return ss_size > get_sigframe_size(); + + return true; +} +#endif /* CONFIG_DYNAMIC_SIGFRAME */ + #ifdef CONFIG_X86_X32_ABI COMPAT_SYSCALL_DEFINE0(x32_rt_sigreturn) { -- 2.25.1