Commit 1bdda24c authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Borislav Petkov
Browse files

signal: Add an optional check for altstack size 



New x86 FPU features will be very large, requiring ~10k of stack in
signal handlers.  These new features require a new approach called
"dynamic features".

The kernel currently tries to ensure that altstacks are reasonably
sized. Right now, on x86, sys_sigaltstack() requires a size of >=2k.
However, that 2k is a constant. Simply raising that 2k requirement
to >10k for the new features would break existing apps which have a
compiled-in size of 2k.

Instead of universally enforcing a larger stack, prohibit a process from
using dynamic features without properly-sized altstacks. This must be
enforced in two places:

 * A dynamic feature can not be enabled without an large-enough altstack
   for each process thread.
 * Once a dynamic feature is enabled, any request to install a too-small
   altstack will be rejected

The dynamic feature enabling code must examine each thread in a
process to ensure that the altstacks are large enough. Add a new lock
(sigaltstack_lock()) to ensure that threads can not race and change
their altstack after being examined.

Add the infrastructure in form of a config option and provide empty
stubs for architectures which do not need dynamic altstack size checks.

This implementation will be fleshed out for x86 in a future patch called

  x86/arch_prctl: Add controls for dynamic XSTATE components

  [dhansen: commit message. ]

Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarChang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021225527.10184-2-chang.seok.bae@intel.com
parent 582b01b6

arch/Kconfig

+3 −0
Original line number Diff line number Diff line
@@ -1288,6 +1288,9 @@ config ARCH_HAS_ELFCORE_COMPAT 
config ARCH_HAS_PARANOID_L1D_FLUSH

	bool



config DYNAMIC_SIGFRAME

	bool



source "kernel/gcov/Kconfig"



source "scripts/gcc-plugins/Kconfig"

include/linux/signal.h

+6 −0
Original line number Diff line number Diff line
@@ -464,6 +464,12 @@ int __save_altstack(stack_t __user *, unsigned long); 
	unsafe_put_user(t->sas_ss_size, &__uss->ss_size, label); \

} while (0);



#ifdef CONFIG_DYNAMIC_SIGFRAME

bool sigaltstack_size_valid(size_t ss_size);

#else

static inline bool sigaltstack_size_valid(size_t size) { return true; }

#endif /* !CONFIG_DYNAMIC_SIGFRAME */



#ifdef CONFIG_PROC_FS

struct seq_file;

extern void render_sigset_t(struct seq_file *, const char *, sigset_t *);

kernel/signal.c

+29 −6
Original line number Diff line number Diff line
@@ -4151,11 +4151,29 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact) 
	return 0;

}



#ifdef CONFIG_DYNAMIC_SIGFRAME

static inline void sigaltstack_lock(void)

	__acquires(&current->sighand->siglock)

{

	spin_lock_irq(&current->sighand->siglock);

}



static inline void sigaltstack_unlock(void)

	__releases(&current->sighand->siglock)

{

	spin_unlock_irq(&current->sighand->siglock);

}

#else

static inline void sigaltstack_lock(void) { }

static inline void sigaltstack_unlock(void) { }

#endif



static int

do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp,

		size_t min_ss_size)

{

	struct task_struct *t = current;

	int ret = 0;



	if (oss) {

		memset(oss, 0, sizeof(stack_t));
@@ -4179,19 +4197,24 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp, 
				ss_mode != 0))

			return -EINVAL;



		sigaltstack_lock();

		if (ss_mode == SS_DISABLE) {

			ss_size = 0;

			ss_sp = NULL;

		} else {

			if (unlikely(ss_size < min_ss_size))

				return -ENOMEM;

				ret = -ENOMEM;

			if (!sigaltstack_size_valid(ss_size))

				ret = -ENOMEM;

		}



		if (!ret) {

			t->sas_ss_sp = (unsigned long) ss_sp;

			t->sas_ss_size = ss_size;

			t->sas_ss_flags = ss_flags;

		}

	return 0;

		sigaltstack_unlock();

	}

	return ret;

}



SYSCALL_DEFINE2(sigaltstack,const stack_t __user *,uss, stack_t __user *,uoss)