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Subject[PATCH 4.14 40/75] x86/entry: Use SYSCALL_DEFINE() macros for sys_modify_ldt()
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dave Hansen <dave.hansen@linux.intel.com>


[ Upstream commit da20ab35180780e4a6eadc804544f1fa967f3567 ]

We do not have tracepoints for sys_modify_ldt() because we define
it directly instead of using the normal SYSCALL_DEFINEx() macros.

However, there is a reason sys_modify_ldt() does not use the macros:
it has an 'int' return type instead of 'unsigned long'. This is
a bug, but it's a bug cemented in the ABI.

What does this mean? If we return -EINVAL from a function that
returns 'int', we have 0x00000000ffffffea in %rax. But, if we
return -EINVAL from a function returning 'unsigned long', we end
up with 0xffffffffffffffea in %rax, which is wrong.

To work around this and maintain the 'int' behavior while using
the SYSCALL_DEFINEx() macros, so we add a cast to 'unsigned int'
in both implementations of sys_modify_ldt().

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Brian Gerst <brgerst@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20171018172107.1A79C532@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/include/asm/syscalls.h | 2 +-
arch/x86/kernel/ldt.c | 16 +++++++++++++---
arch/x86/um/ldt.c | 7 +++++--
3 files changed, 19 insertions(+), 6 deletions(-)

--- a/arch/x86/include/asm/syscalls.h
+++ b/arch/x86/include/asm/syscalls.h
@@ -21,7 +21,7 @@ asmlinkage long sys_ioperm(unsigned long
asmlinkage long sys_iopl(unsigned int);

/* kernel/ldt.c */
-asmlinkage int sys_modify_ldt(int, void __user *, unsigned long);
+asmlinkage long sys_modify_ldt(int, void __user *, unsigned long);

/* kernel/signal.c */
asmlinkage long sys_rt_sigreturn(void);
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -13,6 +13,7 @@
#include <linux/string.h>
#include <linux/mm.h>
#include <linux/smp.h>
+#include <linux/syscalls.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/uaccess.h>
@@ -295,8 +296,8 @@ out:
return error;
}

-asmlinkage int sys_modify_ldt(int func, void __user *ptr,
- unsigned long bytecount)
+SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
+ unsigned long , bytecount)
{
int ret = -ENOSYS;

@@ -314,5 +315,14 @@ asmlinkage int sys_modify_ldt(int func,
ret = write_ldt(ptr, bytecount, 0);
break;
}
- return ret;
+ /*
+ * The SYSCALL_DEFINE() macros give us an 'unsigned long'
+ * return type, but tht ABI for sys_modify_ldt() expects
+ * 'int'. This cast gives us an int-sized value in %rax
+ * for the return code. The 'unsigned' is necessary so
+ * the compiler does not try to sign-extend the negative
+ * return codes into the high half of the register when
+ * taking the value from int->long.
+ */
+ return (unsigned int)ret;
}
--- a/arch/x86/um/ldt.c
+++ b/arch/x86/um/ldt.c
@@ -6,6 +6,7 @@
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/slab.h>
+#include <linux/syscalls.h>
#include <linux/uaccess.h>
#include <asm/unistd.h>
#include <os.h>
@@ -369,7 +370,9 @@ void free_ldt(struct mm_context *mm)
mm->arch.ldt.entry_count = 0;
}

-int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount)
+SYSCALL_DEFINE3(modify_ldt, int , func , void __user * , ptr ,
+ unsigned long , bytecount)
{
- return do_modify_ldt_skas(func, ptr, bytecount);
+ /* See non-um modify_ldt() for why we do this cast */
+ return (unsigned int)do_modify_ldt_skas(func, ptr, bytecount);
}

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