[SRU][Xenial][PATCH 1/2] signal: Introduce COMPAT_SIGMINSTKSZ for use in compat_sys_sigaltstack

[Juerg Haefliger]

From: Will Deacon <will.deacon at arm.com>

https://bugs.launchpad.net/bugs/1844155

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

(backported from commit 22839869f21ab3850fbbac9b425ccc4c0023926f)
[juergh: Adjusted context.]
Signed-off-by: Juerg Haefliger <juergh at canonical.com>
---
 include/linux/compat.h |  3 +++
 kernel/signal.c        | 14 +++++++++-----
 2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/include/linux/compat.h b/include/linux/compat.h
index a76c9172b2eb..1e1348b64d5c 100644
--- a/include/linux/compat.h
+++ b/include/linux/compat.h
@@ -65,6 +65,9 @@ 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)
diff --git a/kernel/signal.c b/kernel/signal.c
index 072fd152ab01..b4fa1d864a79 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -3170,7 +3170,8 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
 }
 
 static int
-do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long sp)
+do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long sp,
+		size_t min_ss_size)
 {
 	stack_t oss;
 	int error;
@@ -3213,7 +3214,7 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s
 			ss_sp = NULL;
 		} else {
 			error = -ENOMEM;
-			if (ss_size < MINSIGSTKSZ)
+			if (ss_size < min_ss_size)
 				goto out;
 		}
 
@@ -3236,12 +3237,14 @@ out:
 }
 SYSCALL_DEFINE2(sigaltstack,const stack_t __user *,uss, stack_t __user *,uoss)
 {
-	return do_sigaltstack(uss, uoss, current_user_stack_pointer());
+	return do_sigaltstack(uss, uoss, current_user_stack_pointer(),
+			      MINSIGSTKSZ);
 }
 
 int restore_altstack(const stack_t __user *uss)
 {
-	int err = do_sigaltstack(uss, NULL, current_user_stack_pointer());
+	int err = do_sigaltstack(uss, NULL, current_user_stack_pointer(),
+				 MINSIGSTKSZ);
 	/* squash all but EFAULT for now */
 	return err == -EFAULT ? err : 0;
 }
@@ -3277,7 +3280,8 @@ COMPAT_SYSCALL_DEFINE2(sigaltstack,
 	set_fs(KERNEL_DS);
 	ret = do_sigaltstack((stack_t __force __user *) (uss_ptr ? &uss : NULL),
 			     (stack_t __force __user *) &uoss,
-			     compat_user_stack_pointer());
+			     compat_user_stack_pointer(),
+			     COMPAT_MINSIGSTKSZ);
 	set_fs(seg);
 	if (ret >= 0 && uoss_ptr)  {
 		if (!access_ok(VERIFY_WRITE, uoss_ptr, sizeof(compat_stack_t)) ||
-- 
2.20.1