[RFC 7/9] x86/mm: Handle tagged memory accesses from kernel threads

[Kirill A. Shutemov]

When a kernel thread performs memory access on behalf of a process (like
in async I/O, io_uring, etc.) it has to respect tagging setup of the
process as user addresses can include tags.

Normally, LAM setup is per-thread and recorded in thread flags, but for
this use case we also track LAM setup per-mm. mm->context.lam would
record LAM that allows the most tag bits among the threads of the mm.

The info used by switch_mm_irqs_off() to construct CR3 if the task is
kernel thread. Thread flags of the kernel thread get updated according
to mm->context.lam. It allows untagged_addr() to work correctly.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/mmu.h |  1 +
 arch/x86/mm/tlb.c          | 28 ++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h
index 9257667d13c5..fb05d6a11538 100644
--- a/arch/x86/include/asm/mmu.h
+++ b/arch/x86/include/asm/mmu.h
@@ -35,6 +35,7 @@ typedef struct {
 #ifdef CONFIG_X86_64
 	/* True if mm supports a task running in 32 bit compatibility mode. */
 	unsigned short ia32_compat;
+	u8 lam;
 #endif
 
 	struct mutex lock;
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 138d4748aa97..1f9749da12e4 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -176,6 +176,34 @@ static u8 gen_lam(struct task_struct *tsk, struct mm_struct *mm)
 	if (!tsk)
 		return LAM_NONE;
 
+	if (tsk->flags & PF_KTHREAD) {
+		/*
+		 * For kernel thread use the most permissive LAM
+		 * used by the mm. It's required to handle kernel thread
+		 * memory accesses on behalf of a process.
+		 *
+		 * Adjust thread flags accodringly, so untagged_addr() would
+		 * work correctly.
+		 */
+		switch (mm->context.lam) {
+		case LAM_NONE:
+			clear_thread_flag(TIF_LAM_U48);
+			clear_thread_flag(TIF_LAM_U57);
+			return LAM_NONE;
+		case LAM_U57:
+			clear_thread_flag(TIF_LAM_U48);
+			set_thread_flag(TIF_LAM_U57);
+			return LAM_U57;
+		case LAM_U48:
+			set_thread_flag(TIF_LAM_U48);
+			clear_thread_flag(TIF_LAM_U57);
+			return LAM_U48;
+		default:
+			WARN_ON_ONCE(1);
+			return LAM_NONE;
+		}
+	}
+
 	if (test_ti_thread_flag(ti, TIF_LAM_U57))
 		return LAM_U57;
 	if (test_ti_thread_flag(ti, TIF_LAM_U48))
-- 
2.26.2