linux-kernel - [PATCH v1 1/2] uaccess: Add __user_write_access

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Message-ID: <20251023000535.2897002-2-kuniyu@google.com>
Date: Thu, 23 Oct 2025 00:04:43 +0000
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, Paul Walmsley <pjw@...nel.org>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Jens Axboe <axboe@...nel.dk>, 
	Christian Brauner <brauner@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Nicholas Piggin <npiggin@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Alexandre Ghiti <alex@...ti.fr>, "H. Peter Anvin" <hpa@...or.com>, Eric Dumazet <edumazet@...gle.com>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Kuniyuki Iwashima <kuni1840@...il.com>, x86@...nel.org, 
	linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH v1 1/2] uaccess: Add __user_write_access_begin().

In epoll_wait(2), ep_check_params() performs a bulk check for
the passed user address:

  if (!access_ok(evs, maxevents * sizeof(struct epoll_event)))

And later, epoll_put_uevent() uses __put_user() twice to copy
2 data into the region.

unsafe_put_user() can be used to save a stac/clac pair, but
masked_user_access_begin() or user_access_begin() introduces
an unnecessary address masking or access_ok().

Add a low-level helper for such a use case.

Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>
---
 arch/arm64/include/asm/uaccess.h   |  1 +
 arch/powerpc/include/asm/uaccess.h | 13 ++++++++++---
 arch/riscv/include/asm/uaccess.h   |  1 +
 arch/x86/include/asm/uaccess.h     |  1 +
 include/linux/uaccess.h            |  1 +
 5 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 1aa4ecb73429..30726ce182cb 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -422,6 +422,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
 }
 #define user_access_begin(a,b)	user_access_begin(a,b)
 #define user_access_end()	uaccess_ttbr0_disable()
+#define __user_write_access_begin(a,b)	uaccess_ttbr0_enable()
 #define unsafe_put_user(x, ptr, label) \
 	__raw_put_mem("sttr", x, uaccess_mask_ptr(ptr), label, U)
 #define unsafe_get_user(x, ptr, label) \
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 4f5a46a77fa2..910bf469128d 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -437,15 +437,22 @@ user_read_access_begin(const void __user *ptr, size_t len)
 #define user_read_access_begin	user_read_access_begin
 #define user_read_access_end		prevent_current_read_from_user
 
+static __always_inline void
+__user_write_access_begin(const void __user *ptr, size_t len)
+{
+	might_fault();
+
+	allow_write_to_user((void __user *)ptr, len);
+}
+#define __user_write_access_begin	__user_write_access_begin
+
 static __must_check __always_inline bool
 user_write_access_begin(const void __user *ptr, size_t len)
 {
 	if (unlikely(!access_ok(ptr, len)))
 		return false;
 
-	might_fault();
-
-	allow_write_to_user((void __user *)ptr, len);
+	__user_write_access_begin(ptr, len);
 	return true;
 }
 #define user_write_access_begin	user_write_access_begin
diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index f5f4f7f85543..9adc8f0dd1c8 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -452,6 +452,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
 }
 #define user_access_begin user_access_begin
 #define user_access_end __disable_user_access
+#define __user_write_access_begin(a,b) __enable_user_access()
 
 static inline unsigned long user_access_save(void) { return 0UL; }
 static inline void user_access_restore(unsigned long enabled) { }
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 91a3fb8ae7ff..23edbaef9f71 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -524,6 +524,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
 }
 #define user_access_begin(a,b)	user_access_begin(a,b)
 #define user_access_end()	__uaccess_end()
+#define __user_write_access_begin(a,b)	__uaccess_begin()
 
 #define user_access_save()	smap_save()
 #define user_access_restore(x)	smap_restore(x)
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 1beb5b395d81..a6e32784e6cd 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -552,6 +552,7 @@ do {							\
 #ifndef user_access_begin
 #define user_access_begin(ptr,len) access_ok(ptr, len)
 #define user_access_end() do { } while (0)
+#define __user_write_access_begin(ptr,len) do { } while (0)
 #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
 #define unsafe_get_user(x,p,e) unsafe_op_wrap(__get_user(x,p),e)
 #define unsafe_put_user(x,p,e) unsafe_op_wrap(__put_user(x,p),e)
-- 
2.51.1.814.gb8fa24458f-goog