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From: Christophe Leroy <>
To: Benjamin Herrenschmidt <>,
	Paul Mackerras <>,
	Michael Ellerman <>,,,,,,,
Subject: [PATCH v2 1/5] uaccess: Add user_read_access_begin/end and user_write_access_begin/end
Date: Fri,  3 Apr 2020 07:20:50 +0000 (UTC)	[thread overview]
Message-ID: <> (raw)

Some architectures like powerpc64 have the capability to separate
read access and write access protection.
For get_user() and copy_from_user(), powerpc64 only open read access.
For put_user() and copy_to_user(), powerpc64 only open write access.
But when using unsafe_get_user() or unsafe_put_user(),
user_access_begin open both read and write.

Other architectures like powerpc book3s 32 bits only allow write
access protection. And on this architecture protection is an heavy
operation as it requires locking/unlocking per segment of 256Mbytes.
On those architecture it is therefore desirable to do the unlocking
only for write access. (Note that book3s/32 ranges from very old
powermac from the 90's with powerpc 601 processor, till modern
ADSL boxes with PowerQuicc II processors for instance so it
is still worth considering.)

In order to avoid any risk based of hacking some variable parameters
passed to user_access_begin/end that would allow hacking and
leaving user access open or opening too much, it is preferable to
use dedicated static functions that can't be overridden.

Add a user_read_access_begin and user_read_access_end to only open
read access.

Add a user_write_access_begin and user_write_access_end to only open
write access.

By default, when undefined, those new access helpers default on the
existing user_access_begin and user_access_end.

Signed-off-by: Christophe Leroy <>
Reviewed-by: Kees Cook <>
v2: no change in this patch. See each patch for related changes.

v1 at

This series is based on the discussion we had in January, see . I tried to
take into account all remarks, especially @hpa 's remark to use
a fixed API on not base the relocking on a magic id returned at

This series is awaited for implementing selective lkdtm test to
test powerpc64 independant read and write protection, see

Signed-off-by: Christophe Leroy <>
 include/linux/uaccess.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 67f016010aad..9861c89f93be 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -378,6 +378,14 @@ extern long strnlen_unsafe_user(const void __user *unsafe_addr, long count);
 static inline unsigned long user_access_save(void) { return 0UL; }
 static inline void user_access_restore(unsigned long flags) { }
+#ifndef user_write_access_begin
+#define user_write_access_begin user_access_begin
+#define user_write_access_end user_access_end
+#ifndef user_read_access_begin
+#define user_read_access_begin user_access_begin
+#define user_read_access_end user_access_end
 void usercopy_warn(const char *name, const char *detail, bool to_user,

             reply	other threads:[~2020-04-03  7:20 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-03  7:20 Christophe Leroy [this message]
2020-04-03  7:20 ` [PATCH v2 2/5] uaccess: Selectively open read or write user access Christophe Leroy
2020-05-29  4:20   ` Michael Ellerman
2020-04-03  7:20 ` [PATCH v2 3/5] drm/i915/gem: Replace user_access_begin by user_write_access_begin Christophe Leroy
2020-05-29  4:20   ` Michael Ellerman
2020-04-03  7:20 ` [PATCH v2 4/5] powerpc/uaccess: Implement user_read_access_begin and user_write_access_begin Christophe Leroy
2020-05-29  4:24   ` Michael Ellerman
2020-04-03  7:20 ` [PATCH v2 5/5] uaccess: Rename user_access_begin/end() to user_full_access_begin/end() Christophe Leroy
2020-04-03 18:01   ` Linus Torvalds
2020-04-03 20:52     ` Al Viro
2020-04-21  2:49       ` Al Viro
2020-04-21  9:12         ` Russell King - ARM Linux admin
2020-04-21 18:34         ` Linus Torvalds
2020-04-05 18:47     ` Christophe Leroy
2020-04-04  6:20   ` kbuild test robot
2020-04-04  7:17   ` kbuild test robot
2020-05-29  4:20 ` [PATCH v2 1/5] uaccess: Add user_read_access_begin/end and user_write_access_begin/end Michael Ellerman

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