From: Andy Lutomirski <luto@xxxxxxxxxx> [ Upstream commit 07e1d88adaaeab247b300926f78cc3f950dbeda3 ] On 64-bit kernels ptrace can read the FS/GS base using the register access APIs (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 such as GDB. [ chang: Rebased and revised patch description. ] [ mingo: Revised the changelog some more. ] Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx> Cc: Borislav Petkov <bp@xxxxxxxxx> Cc: Brian Gerst <brgerst@xxxxxxxxx> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Markus T Metzger <markus.t.metzger@xxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Ravi Shankar <ravi.v.shankar@xxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/1537312139-5580-2-git-send-email-chang.seok.bae@xxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- arch/x86/kernel/ptrace.c | 62 +++++++++++++++++++++++++++++++++------- 1 file changed, 52 insertions(+), 10 deletions(-) diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c index 584cdd475bb31..734549492a18b 100644 --- a/arch/x86/kernel/ptrace.c +++ b/arch/x86/kernel/ptrace.c @@ -40,6 +40,7 @@ #include <asm/hw_breakpoint.h> #include <asm/traps.h> #include <asm/syscall.h> +#include <asm/mmu_context.h> #include "tls.h" @@ -343,6 +344,49 @@ static int set_segment_reg(struct task_struct *task, return 0; } +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; +} + #endif /* CONFIG_X86_32 */ static unsigned long get_flags(struct task_struct *task) @@ -436,18 +480,16 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset) #ifdef CONFIG_X86_64 case offsetof(struct user_regs_struct, fs_base): { - /* - * XXX: This will not behave as expected if called on - * current or if fsindex != 0. - */ - return task->thread.fsbase; + if (task->thread.fsindex == 0) + return task->thread.fsbase; + else + return task_seg_base(task, task->thread.fsindex); } case offsetof(struct user_regs_struct, gs_base): { - /* - * XXX: This will not behave as expected if called on - * current or if fsindex != 0. - */ - return task->thread.gsbase; + if (task->thread.gsindex == 0) + return task->thread.gsbase; + else + return task_seg_base(task, task->thread.gsindex); } #endif } -- 2.20.1
