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SubjectRE: [regression in -rc1] Re: [PATCH v6 2/8] x86/fsgsbase/64: Introduce FS/GS base helper functions
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On Wed, Oct 24, 2018 at 12:43 PM Andy Lutomirski <luto@kernel.org> wrote:
> I think x86_fsbase_write_task() makes plenty of sense, but I think
> that callers need to be aware that the effect of writing a nonzero
> fsbase *and* a nonzero fsindex is bizarre on non-FSGSBASE systems. So
> that code should go in the callers. The oddities involved have little
> to do with whether the caller is writing to current or to something
> else.
>
> Arguably the code should be entirely split out into the code that
> writes current (arch_prctl()) and the code that writes a stopped task
> (ptrace). I don't think there are any code paths that genuinely can
> write either.
>

Can you check this patch is close to what in your mind?

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index d6674a425714..5f986e15842e 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -341,19 +341,11 @@ static unsigned long x86_fsgsbase_read_task(struct task_struct *task,

void x86_fsbase_write_cpu(unsigned long fsbase)
{
- /*
- * Set the selector to 0 as a notion, that the segment base is
- * overwritten, which will be checked for skipping the segment load
- * during context switch.
- */
- loadseg(FS, 0);
wrmsrl(MSR_FS_BASE, fsbase);
}

void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
{
- /* Set the selector to 0 for the same reason as %fs above. */
- loadseg(GS, 0);
wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
}

@@ -361,9 +353,7 @@ unsigned long x86_fsbase_read_task(struct task_struct *task)
{
unsigned long fsbase;

- if (task == current)
- fsbase = x86_fsbase_read_cpu();
- else if (task->thread.fsindex == 0)
+ if (task->thread.fsindex == 0)
fsbase = task->thread.fsbase;
else
fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);
@@ -375,9 +365,7 @@ unsigned long x86_gsbase_read_task(struct task_struct *task)
{
unsigned long gsbase;

- if (task == current)
- gsbase = x86_gsbase_read_cpu_inactive();
- else if (task->thread.gsindex == 0)
+ if (task->thread.gsindex == 0)
gsbase = task->thread.gsbase;
else
gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);
@@ -396,8 +384,6 @@ int x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase)

preempt_disable();
task->thread.fsbase = fsbase;
- if (task == current)
- x86_fsbase_write_cpu(fsbase);
task->thread.fsindex = 0;
preempt_enable();

@@ -411,8 +397,6 @@ int x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase)

preempt_disable();
task->thread.gsbase = gsbase;
- if (task == current)
- x86_gsbase_write_cpu_inactive(gsbase);
task->thread.gsindex = 0;
preempt_enable();

@@ -761,20 +745,42 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
switch (option) {
case ARCH_SET_GS: {
ret = x86_gsbase_write_task(task, arg2);
+ if (task == current && ret == 0) {
+ preempt_disable();
+ loadseg(GS, 0);
+ x86_gsbase_write_cpu_inactive();
+ preempt_enable();
+ }
break;
}
case ARCH_SET_FS: {
ret = x86_fsbase_write_task(task, arg2);
+ if (task == current && ret == 0) {
+ preempt_disable();
+ loadseg(FS, 0);
+ x86_fsbase_write_cpu();
+ preempt_enable();
+ }
break;
}
case ARCH_GET_FS: {
- unsigned long base = x86_fsbase_read_task(task);
+ unsigned long base;
+
+ if (task == current)
+ base = x86_fsbase_read_cpu();
+ else
+ base = x86_fsbase_read_task(task);

ret = put_user(base, (unsigned long __user *)arg2);
break;
}
case ARCH_GET_GS: {
- unsigned long base = x86_gsbase_read_task(task);
+ unsigned long base;
+
+ if (task == current)
+ base = x86_gsbase_read_cpu_inactive();
+ else
+ base = x86_gsbase_read_task(task);

ret = put_user(base, (unsigned long __user *)arg2);
break;
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