From: Andy Lutomirski <l...@kernel.org> ptrace can read FS/GS base using the register access API (PTRACE_PEEKUSER, etc) or PTRACE_ARCH_PRCTL. Make both of these mechanisms return the actual FS/GS base.
This will improve debuggability by providing the correct information to ptracer (GDB and etc). Signed-off-by: Andy Lutomirski <l...@kernel.org> [chang: Rebase and revise patch description] Signed-off-by: Chang S. Bae <chang.seok....@intel.com> Reviewed-by: Andi Kleen <a...@linux.intel.com> Cc: H. Peter Anvin <h...@zytor.com> Cc: Dave Hansen <dave.han...@linux.intel.com> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Ingo Molnar <mi...@kernel.org> --- arch/x86/kernel/process_64.c | 67 +++++++++++++++++++++++++++++++++----------- 1 file changed, 51 insertions(+), 16 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index ace0158..e498671 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -279,6 +279,49 @@ static __always_inline void load_seg_legacy(unsigned short prev_index, } } +static unsigned long task_seg_base(struct task_struct *task, + unsigned short selector) +{ + unsigned short idx = selector >> 3; + unsigned long base; + + if (likely((selector & SEGMENT_TI_MASK) == 0)) { + if (unlikely(idx >= GDT_ENTRIES)) + return 0; + + /* + * There are no user segments in the GDT with nonzero bases + * other than the TLS segments. + */ + if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX) + return 0; + + idx -= GDT_ENTRY_TLS_MIN; + base = get_desc_base(&task->thread.tls_array[idx]); + } else { +#ifdef CONFIG_MODIFY_LDT_SYSCALL + struct ldt_struct *ldt; + + /* + * If performance here mattered, we could protect the LDT + * with RCU. This is a slow path, though, so we can just + * take the mutex. + */ + mutex_lock(&task->mm->context.lock); + ldt = task->mm->context.ldt; + if (unlikely(idx >= ldt->nr_entries)) + base = 0; + else + base = get_desc_base(ldt->entries + idx); + mutex_unlock(&task->mm->context.lock); +#else + base = 0; +#endif + } + + return base; +} + void write_fsbase(unsigned long fsbase) { /* set the selector to 0 to not confuse __switch_to */ @@ -297,16 +340,12 @@ unsigned long read_task_fsbase(struct task_struct *task) { unsigned long fsbase; - if (task == current) { + if (task == current) fsbase = read_fsbase(); - } else { - /* - * XXX: This will not behave as expected if called - * if fsindex != 0. This preserves an existing bug - * that will be fixed. - */ + else if (task->thread.fsindex == 0) fsbase = task->thread.fsbase; - } + else + fsbase = task_seg_base(task, task->thread.fsindex); return fsbase; } @@ -315,16 +354,12 @@ unsigned long read_task_gsbase(struct task_struct *task) { unsigned long gsbase; - if (task == current) { + if (task == current) gsbase = read_inactive_gsbase(); - } else { - /* - * XXX: This will not behave as expected if called - * if gsindex != 0. Same bug preservation as above - * read_task_fsbase. - */ + else if (task->thread.gsindex == 0) gsbase = task->thread.gsbase; - } + else + gsbase = task_seg_base(task, task->thread.gsindex); return gsbase; } -- 2.7.4