[PATCH AUTOSEL 4.19 136/209] x86/fsgsbase/64: Fix ptrace() to read the FS/GS base accurately

Sasha Levin
2019-11-13T01:54:14Z

From: Andy Lutomirski <luto@kernel.org>

[ 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@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1537312139-5580-2-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 516ec7586a5fb..8d4d506453106 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