signal: Introduce COMPAT_SIGMINSTKSZ for use in compat_sys_sigaltstack

The sigaltstack(2) system call fails with -ENOMEM if the new alternative
signal stack is found to be smaller than SIGMINSTKSZ. On architectures
such as arm64, where the native value for SIGMINSTKSZ is larger than
the compat value, this can result in an unexpected error being reported
to a compat task. See, for example:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=904385

This patch fixes the problem by extending do_sigaltstack to take the
minimum signal stack size as an additional parameter, allowing the
native and compat system call entry code to pass in their respective
values. COMPAT_SIGMINSTKSZ is just defined as SIGMINSTKSZ if it has not
been defined by the architecture.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Oleg Nesterov <oleg@redhat.com>
Reported-by: Steve McIntyre <steve.mcintyre@arm.com>
Tested-by: Steve McIntyre <93sam@debian.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

include/linux/compat.h

@@ -103,6 +103,9 @@ typedef struct compat_sigaltstack {
 	compat_size_t			ss_size;
 } compat_stack_t;
 #endif
+#ifndef COMPAT_MINSIGSTKSZ
+#define COMPAT_MINSIGSTKSZ	MINSIGSTKSZ
+#endif
 
 #define compat_jiffies_to_clock_t(x)	\
 		(((unsigned long)(x) * COMPAT_USER_HZ) / HZ)

kernel/signal.c

@@ -3460,7 +3460,8 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
 }
 
 static int
-do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp)
+do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp,
+		size_t min_ss_size)
 {
 	struct task_struct *t = current;
 
@@ -3490,7 +3491,7 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp)
 			ss_size = 0;
 			ss_sp = NULL;
 		} else {
-			if (unlikely(ss_size < MINSIGSTKSZ))
+			if (unlikely(ss_size < min_ss_size))
 				return -ENOMEM;
 		}
 
@@ -3508,7 +3509,8 @@ SYSCALL_DEFINE2(sigaltstack,const stack_t __user *,uss, stack_t __user *,uoss)
 	if (uss && copy_from_user(&new, uss, sizeof(stack_t)))
 		return -EFAULT;
 	err = do_sigaltstack(uss ? &new : NULL, uoss ? &old : NULL,
-			      current_user_stack_pointer());
+			      current_user_stack_pointer(),
+			      MINSIGSTKSZ);
 	if (!err && uoss && copy_to_user(uoss, &old, sizeof(stack_t)))
 		err = -EFAULT;
 	return err;
@@ -3519,7 +3521,8 @@ int restore_altstack(const stack_t __user *uss)
 	stack_t new;
 	if (copy_from_user(&new, uss, sizeof(stack_t)))
 		return -EFAULT;
-	(void)do_sigaltstack(&new, NULL, current_user_stack_pointer());
+	(void)do_sigaltstack(&new, NULL, current_user_stack_pointer(),
+			     MINSIGSTKSZ);
 	/* squash all but EFAULT for now */
 	return 0;
 }
@@ -3553,7 +3556,8 @@ static int do_compat_sigaltstack(const compat_stack_t __user *uss_ptr,
 		uss.ss_size = uss32.ss_size;
 	}
 	ret = do_sigaltstack(uss_ptr ? &uss : NULL, &uoss,
-			     compat_user_stack_pointer());
+			     compat_user_stack_pointer(),
+			     COMPAT_MINSIGSTKSZ);
 	if (ret >= 0 && uoss_ptr)  {
 		compat_stack_t old;
 		memset(&old, 0, sizeof(old));