From: Eric Dumazet <eduma...@google.com>
[ Upstream commit 8ab49526b53d3172d1d8dd03a75c7d1f5bd21239 ]
syzbot found its way in 86_fsgsbase_read_task() and triggered this oops:
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 PID: 6866 Comm: syz-executor262 Not tainted 5.8.0-syzkaller #0
RIP: 0010:x86_fsgsbase_read_task+0x16d/0x310 arch/x86/kernel/process_64.c:393
Call Trace:
putreg32+0x3ab/0x530 arch/x86/kernel/ptrace.c:876
genregs32_set arch/x86/kernel/ptrace.c:1026 [inline]
genregs32_set+0xa4/0x100 arch/x86/kernel/ptrace.c:1006
copy_regset_from_user include/linux/regset.h:326 [inline]
ia32_arch_ptrace arch/x86/kernel/ptrace.c:1061 [inline]
compat_arch_ptrace+0x36c/0xd90 arch/x86/kernel/ptrace.c:1198
__do_compat_sys_ptrace kernel/ptrace.c:1420 [inline]
__se_compat_sys_ptrace kernel/ptrace.c:1389 [inline]
__ia32_compat_sys_ptrace+0x220/0x2f0 kernel/ptrace.c:1389
do_syscall_32_irqs_on arch/x86/entry/common.c:84 [inline]
__do_fast_syscall_32+0x57/0x80 arch/x86/entry/common.c:126
do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:149
entry_SYSENTER_compat_after_hwframe+0x4d/0x5c
This can happen if ptrace() or sigreturn() pokes an LDT selector into FS
or GS for a task with no LDT and something tries to read the base before
a return to usermode notices the bad selector and fixes it.
The fix is to make sure ldt pointer is not NULL.
Fixes: 07e1d88adaae ("x86/fsgsbase/64: Fix ptrace() to read the FS/GS base
accurately")
Co-developed-by: Jann Horn <ja...@google.com>
Signed-off-by: Eric Dumazet <eduma...@google.com>
Reported-by: syzbot <syzkal...@googlegroups.com>
Acked-by: Andy Lutomirski <l...@kernel.org>
Cc: Chang S. Bae <chang.seok....@intel.com>
Cc: Andy Lutomirski <l...@amacapital.net>
Cc: Borislav Petkov <b...@alien8.de>
Cc: Brian Gerst <brge...@gmail.com>
Cc: Dave Hansen <dave.han...@linux.intel.com>
Cc: Denys Vlasenko <dvlas...@redhat.com>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Markus T Metzger <markus.t.metz...@intel.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Ravi Shankar <ravi.v.shan...@intel.com>
Cc: Rik van Riel <r...@surriel.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Ingo Molnar <mi...@kernel.org>
Signed-off-by: Linus Torvalds <torva...@linux-foundation.org>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---
arch/x86/kernel/process_64.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index af64519b26957..da3cc3a10d63f 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -316,7 +316,7 @@ static unsigned long x86_fsgsbase_read_task(struct
task_struct *task,
*/
mutex_lock(&task->mm->context.lock);
ldt = task->mm->context.ldt;
- if (unlikely(idx >= ldt->nr_entries))
+ if (unlikely(!ldt || idx >= ldt->nr_entries))
base = 0;
else
base = get_desc_base(ldt->entries + idx);
--
2.25.1
