[tip: x86/cleanups] x86/compat: Simplify compat syscall userspace allocation

[tip-bot2 for Gabriel Krisman Bertazi]

The following commit has been merged into the x86/cleanups branch of tip:

Commit-ID:     214f0e804358cdd13b5cbe4445189f23e30618b4
Gitweb:        https://git.kernel.org/tip/214f0e804358cdd13b5cbe4445189f23e30618b4
Author:        Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
AuthorDate:    Sat, 03 Oct 2020 23:25:28 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Mon, 26 Oct 2020 13:46:46 +01:00

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@xxxxxxxxxx>
Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201004032536.1229030-3-krisman@xxxxxxxxxxxxx

---
 arch/x86/include/asm/compat.h | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/compat.h b/arch/x86/include/asm/compat.h
index 0e327a0..f145e33 100644
--- 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);
 }