[PATCH v16 18/18] arm64: vdso: Expose sigreturn address on vdso to the kernel

[Jinjie Ruan]

Syscall User Dispatch (SUD) requires the signal trampoline code to not be
intercepted. This is necessary to support returning with a locked selector
while avoiding infinite recursion back into the signal handler.

Implement arch_syscall_is_vdso_sigreturn() for arm64 to support this
exclusion mechanism. For native 64-bit tasks, it checks whether the current
PC matches the 'svc #0' instruction inside the vDSO sigreturn trampoline.

	SYM_CODE_START(__kernel_rt_sigreturn)
	        mov     x8, #__NR_rt_sigreturn
	        svc     #0
	SYM_CODE_END(__kernel_rt_sigreturn)

For COMPAT tasks, it verifies if the instruction falls within
the architecture's 'sigpage' range, allowing the kernel to safely bypass
dispatching syscalls originating from these areas back to userspace.

Suggested-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
Suggested-by: kemal <kmal@xxxxxxx>
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
 arch/arm64/include/asm/elf.h |  1 +
 arch/arm64/kernel/vdso.c     | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
index d2779d604c7b..f43d4dea916a 100644
--- a/arch/arm64/include/asm/elf.h
+++ b/arch/arm64/include/asm/elf.h
@@ -185,6 +185,7 @@ do {									\
 struct linux_binprm;
 extern int arch_setup_additional_pages(struct linux_binprm *bprm,
 				       int uses_interp);
+extern bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs);
 
 /* 1GB of VA */
 #ifdef CONFIG_COMPAT
diff --git a/arch/arm64/kernel/vdso.c b/arch/arm64/kernel/vdso.c
index 592dd8668de4..5a0314a3c26e 100644
--- a/arch/arm64/kernel/vdso.c
+++ b/arch/arm64/kernel/vdso.c
@@ -343,3 +343,19 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
 
 	return ret;
 }
+
+bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
+{
+	unsigned long sigtramp;
+
+#ifdef CONFIG_COMPAT
+	if (is_compat_task()) {
+		unsigned long sigpage = (unsigned long)current->mm->context.sigpage;
+
+		return regs->pc >= sigpage && regs->pc < (sigpage + PAGE_SIZE);
+	}
+#endif
+	sigtramp = (unsigned long)VDSO_SYMBOL(current->mm->context.vdso, sigtramp);
+
+	return regs->pc == (sigtramp + 8);
+}
-- 
2.34.1