When FSGSBASE enabled, ptracer's FS/GS selector update
fetches FS/GS base from GDT/LDT. This emulation of FS/GS
segment loading provides backward compatibility for the
legacy ptracers.
When ptracer sets FS/GS selector, its base is going to be
(accordingly) reloaded as the tracee resumes. This is without
FSGSBASE. With FSGSBASE, FS/GS base is preserved regardless
of its selector. Thus, emulating FS/GS load in ptrace is
requested to keep compatible with what has been with FS/GS
setting.
Additionally, whenever a new base value is written, the
FSGSBASE-enabled kernel allows the tracee effectively carry
on. This also means that when both selector and base are
changed, the base is not fetched from GDT/LDT, but
preserved as given.
In a summary, ptracer's update on FS/GS selector and base
yields such results on tracee's base:
- When FS/GS selector only changed (to nonzero), fetch base
from GDT/LDT (legacy behavior)
- When FS/GS base (regardless of selector) changed, tracee
will have the base
Suggested-by: Markus T. Metzger <markus.t.metz...@intel.com>
Suggested-by: H. Peter Anvin <h...@zytor.com>
Signed-off-by: Chang S. Bae <chang.seok....@intel.com>
Cc: Andi Kleen <a...@linux.intel.com>
Cc: Andy Lutomirski <l...@kernel.org>
---
arch/x86/include/asm/fsgsbase.h | 4 +++
arch/x86/kernel/process_64.c | 4 +--
arch/x86/kernel/ptrace.c | 68 +++++++++++++++++++++++++++++++++++------
3 files changed, 65 insertions(+), 11 deletions(-)
diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
index 76d3457..430ae40 100644
--- a/arch/x86/include/asm/fsgsbase.h
+++ b/arch/x86/include/asm/fsgsbase.h
@@ -17,6 +17,10 @@ unsigned long read_task_gsbase(struct task_struct *task);
int write_task_fsbase(struct task_struct *task, unsigned long fsbase);
int write_task_gsbase(struct task_struct *task, unsigned long gsbase);
+/* Read (FS/GS) base from GDT/LDT */
+unsigned long task_seg_base(struct task_struct *task,
+ unsigned short selector);
+
/*
* Must be protected by X86_FEATURE_FSGSBASE check.
*/
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 5aae132..ef32f75 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -328,8 +328,8 @@ static __always_inline void load_fsgs(struct thread_struct
*prev,
}
}
-static unsigned long task_seg_base(struct task_struct *task,
- unsigned short selector)
+unsigned long task_seg_base(struct task_struct *task,
+ unsigned short selector)
{
unsigned short idx = selector >> 3;
unsigned long base;
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index ee37e28..be3e022 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -307,14 +307,26 @@ static int set_segment_reg(struct task_struct *task,
switch (offset) {
case USER_REGS_OFFSET(fs):
- task->thread.fsindex = value;
if (task == current)
- loadsegment(fs, task->thread.fsindex);
+ loadsegment(fs, value);
+ /*
+ * %fs setting goes to reload its base, when tracee
+ * resumes without FSGSBASE (legacy). Here with FSGSBASE
+ * FS base is (manually) fetched from GDT/LDT when needed.
+ */
+ else if (static_cpu_has(X86_FEATURE_FSGSBASE) &&
+ (value != 0) && (task->thread.fsindex != value))
+ task->thread.fsbase = task_seg_base(task, value);
+ task->thread.fsindex = value;
break;
case USER_REGS_OFFSET(gs):
- task->thread.gsindex = value;
if (task == current)
- load_gs_index(task->thread.gsindex);
+ load_gs_index(value);
+ /* Same as %fs handling above */
+ else if (static_cpu_has(X86_FEATURE_FSGSBASE) &&
+ (value != 0) && (task->thread.gsindex != value))
+ task->thread.gsbase = task_seg_base(task, value);
+ task->thread.gsindex = value;
break;
case USER_REGS_OFFSET(ds):
task->thread.ds = value;
@@ -433,14 +445,31 @@ static int putregs(struct task_struct *child,
((offset + count) >= USER_REGS_OFFSET(fs));
bool gs_fully_covered = (offset <= USER_REGS_OFFSET(gs_base)) &&
((offset + count) >= USER_REGS_OFFSET(gs));
+ bool fs_updated = false, gs_updated = false;
offset += count - sizeof(*v);
while (count >= sizeof(*v) && !ret) {
v--;
switch (offset) {
+ case USER_REGS_OFFSET(fs):
+ if (fs_fully_covered &&
+ static_cpu_has(X86_FEATURE_FSGSBASE)) {
+ if (invalid_selector(*v))
+ return -EIO;
+ /*
+ * Set the flag to fetch fsbase from GDT/LDT
+ * with FSGSBASE
+ */
+ fs_updated = (*v != 0) &&
+ (child->thread.fsindex != *v);
+ child->thread.fsindex = *v;
+ break;
+ }
case USER_REGS_OFFSET(fs_base):
if (fs_fully_covered) {
+ struct thread_struct *thread = &child->thread;
+
if (unlikely(*v >= TASK_SIZE_MAX))
return -EIO;
/*
@@ -448,17 +477,38 @@ static int putregs(struct task_struct *child,
* write_task_fsbase() tends to overwrite
* task's %fs. Simply setting base only here.
*/
- if (child->thread.fsbase != *v)
- child->thread.fsbase = *v;
+ if (thread->fsbase != *v)
+ thread->fsbase = *v;
+ else if (fs_updated)
+ thread->fsbase =
+ task_seg_base(child,
+ thread->fsindex);
+ break;
+ }
+ case USER_REGS_OFFSET(gs):
+ if (gs_fully_covered &&
+ static_cpu_has(X86_FEATURE_FSGSBASE)) {
+ if (invalid_selector(*v))
+ return -EIO;
+ /* Same here as the %fs handling above */
+ gs_updated = (*v != 0) &&
+ (child->thread.gsindex != *v);
+ child->thread.gsindex = *v;
break;
}
case USER_REGS_OFFSET(gs_base):
if (gs_fully_covered) {
+ struct thread_struct *thread = &child->thread;
+
if (unlikely(*v >= TASK_SIZE_MAX))
return -EIO;
- /* Same here as the %fs handling above */
- if (child->thread.gsbase != *v)
- child->thread.gsbase = *v;
+ /* Same here as the %fs_base handling above */
+ if (thread->gsbase != *v)
+ thread->gsbase = *v;
+ else if (gs_updated)
+ thread->gsbase =
+ task_seg_base(child,
+ thread->gsindex);
break;
}
default:
--
2.7.4
