x86/compat: Simplify compat syscall userspace allocation

When allocating user memory space for a compat system call, don't consider
whether the originating code is IA32 or X32, just allocate from a safe
region for both, beyond the redzone.  This should be safe for IA32, and has
the benefit of avoiding TIF_IA32, which is about to be removed.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201004032536.1229030-3-krisman@collabora.com

diff --git a/arch/x86/include/asm/compat.h b/arch/x86/include/asm/compat.h
index 0e327a01f50fbbff551727d38ee51fe351011254..f145e3326c6d7318d649e6a423a3f142d1e97bd0 100644 (file)
--- a/arch/x86/include/asm/compat.h
+++ b/arch/x86/include/asm/compat.h
@@ -177,14 +177,13 @@ typedef struct user_regs_struct compat_elf_gregset_t;
 
 static inline void __user *arch_compat_alloc_user_space(long len)
 {
-       compat_uptr_t sp;
-
-       if (test_thread_flag(TIF_IA32)) {
-               sp = task_pt_regs(current)->sp;
-       } else {
-               /* -128 for the x32 ABI redzone */
-               sp = task_pt_regs(current)->sp - 128;
-       }
+       compat_uptr_t sp = task_pt_regs(current)->sp;
+
+       /*
+        * -128 for the x32 ABI redzone.  For IA32, it is not strictly
+        * necessary, but not harmful.
+        */
+       sp -= 128;
 
        return (void __user *)round_down(sp - len, 16);
 }