Handle bogus %cs selector in single-step instruction decoding

The code for LDT segment selectors was not robust in the face of a bogus
selector set in %cs via ptrace before the single-step was done.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 7 deletions

diff --git a/arch/x86_64/kernel/ptrace.c b/arch/x86_64/kernel/ptrace.c
index fa6775ef729f..e83cc67155ac 100644
--- a/arch/x86_64/kernel/ptrace.c
+++ b/arch/x86_64/kernel/ptrace.c
@@ -102,16 +102,25 @@ unsigned long convert_rip_to_linear(struct task_struct *child, struct pt_regs *r
                 u32 *desc;
                 unsigned long base;
 
-                down(&child->mm->context.sem);
-                desc = child->mm->context.ldt + (seg & ~7);
-                base = (desc[0] >> 16) | ((desc[1] & 0xff) << 16) | (desc[1] & 0xff000000);
+                seg &= ~7UL;
 
-                /* 16-bit code segment? */
-                if (!((desc[1] >> 22) & 1))
-                        addr &= 0xffff;
-                addr += base;
+                down(&child->mm->context.sem);
+                if (unlikely((seg >> 3) >= child->mm->context.size))
+                        addr = -1L; /* bogus selector, access would fault */
+                else {
+                        desc = child->mm->context.ldt + seg;
+                        base = ((desc[0] >> 16) |
+                                ((desc[1] & 0xff) << 16) |
+                                (desc[1] & 0xff000000));
+
+                        /* 16-bit code segment? */
+                        if (!((desc[1] >> 22) & 1))
+                                addr &= 0xffff;
+                        addr += base;
+                }
                 up(&child->mm->context.sem);
         }
+
         return addr;
 }