arm64: compat: Split the sigreturn trampolines and kuser helpers (C sources)

From: Kevin Brodsky <[email protected]>

AArch32 processes are currently installed a special [vectors] page that contains the sigreturn trampolines and the kuser helpers, at the fixed address mandated by the kuser helpers ABI.

Having both functionalities in the same page has become problematic, because:

* It makes it impossible to disable the kuser helpers (the sigreturn trampolines cannot be removed), which is possible on arm.

* A future 32-bit vDSO would provide the sigreturn trampolines itself, making those in [vectors] redundant.

This patch addresses the problem by moving the sigreturn trampolines to a separate [sigpage] page, mirroring [sigpage] on arm.

Even though [vectors] has always been a misnomer on arm64/compat, as there is no AArch32 vector there (and now only the kuser helpers), its name has been left unchanged, for compatibility with arm (there are reports of software relying on [vectors] being there as the last mapping in /proc/maps).

mm->context.vdso used to point to the [vectors] page, which is unnecessary (as its address is fixed). It now points to the [sigpage] page (whose address is randomized like a vDSO).

v2:
- reduce churniness (and defer later to vDSO patches)
- vectors_page and compat_vdso_spec as array of 2
- free sigpage if vectors allocation failed

v3:
- rebase

---
 arch/arm64/include/asm/processor.h |  4 +-
 arch/arm64/include/asm/signal32.h  |  2 -
 arch/arm64/kernel/signal32.c       |  5 +--
 arch/arm64/kernel/vdso.c           | 82 ++++++++++++++++++++++++++------------
 4 files changed, 60 insertions(+), 33 deletions(-)

index 29adab8138c3..a61609eca814 100644
@@ -39,9 +39,9 @@
 
 #define STACK_TOP_MAX		TASK_SIZE_64
 #ifdef CONFIG_COMPAT
-#define AARCH32_VECTORS_BASE	0xffff0000
+#define AARCH32_KUSER_HELPERS_BASE 0xffff0000
 #define STACK_TOP		(test_thread_flag(TIF_32BIT) ? \
-				AARCH32_VECTORS_BASE : STACK_TOP_MAX)
+				AARCH32_KUSER_HELPERS_BASE : STACK_TOP_MAX)
 #else
 #define STACK_TOP		STACK_TOP_MAX
 #endif /* CONFIG_COMPAT */

index 81abea0b7650..58e288aaf0ba 100644
@@ -20,8 +20,6 @@
 #ifdef CONFIG_COMPAT
 #include <linux/compat.h>
 
-#define AARCH32_KERN_SIGRET_CODE_OFFSET	0x500
-
 int compat_setup_frame(int usig, struct ksignal *ksig, sigset_t *set,
 		       struct pt_regs *regs);
 int compat_setup_rt_frame(int usig, struct ksignal *ksig, sigset_t *set,

index 4e5a664be04b..bde7acb53c74 100644
@@ -484,14 +484,13 @@ static void compat_setup_return(struct pt_regs *regs, struct k_sigaction *ka,
 		retcode = ptr_to_compat(ka->sa.sa_restorer);
 	} else {
 		/* Set up sigreturn pointer */
+		void *sigreturn_base = current->mm->context.vdso;
 		unsigned int idx = thumb << 1;
 
 		if (ka->sa.sa_flags & SA_SIGINFO)
 			idx += 3;
 
-		retcode = AARCH32_VECTORS_BASE +
-			  AARCH32_KERN_SIGRET_CODE_OFFSET +
-			  (idx << 2) + thumb;
+		retcode = ptr_to_compat(sigreturn_base) + (idx << 2) + thumb;
 	}
 
 	regs->regs[0]	= usig;

2.14.1.581.gf28d330327-goog