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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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211021225527.10184-2-chang.seok.bae@intel.com
This commit is contained in:
Thomas Gleixner 2021-10-21 15:55:05 -07:00 committed by Borislav Petkov
parent 582b01b6ab
commit 1bdda24c4a
3 changed files with 38 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
arch/Kconfig
View File

@ -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"

6
include/linux/signal.h
View File

@ -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 *);

35
kernel/signal.c
View File

@ -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;
}
t->sas_ss_sp = (unsigned long) ss_sp;
t->sas_ss_size = ss_size;
t->sas_ss_flags = ss_flags;
if (!ret) {
t->sas_ss_sp = (unsigned long) ss_sp;
t->sas_ss_size = ss_size;
t->sas_ss_flags = ss_flags;
}
sigaltstack_unlock();
}
return 0;
return ret;
}
SYSCALL_DEFINE2(sigaltstack,const stack_t __user *,uss, stack_t __user *,uoss)