[PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Eric Dumazet]

syzbot found its way in 86_fsgsbase_read_task() [1]

Fix is to make sure ldt pointer is not NULL.

[1]
general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
CPU: 0 PID: 6866 Comm: syz-executor262 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:x86_fsgsbase_read_task+0x16d/0x310 arch/x86/kernel/process_64.c:393
Code: 3c 02 00 0f 85 7c 01 00 00 4c 8b a5 80 04 00 00 44 0f b7 eb 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 08 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 30 01 00 00 45 3b 6c 24 08 0f
RSP: 0018:ffffc90001677db8 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000008 RDI: 0000000000000008
RBP: ffff88809eaf9780 R08: 0000000000000000 R09: ffff88809eaf9c17
R10: ffffed1013d5f382 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000020000080
FS:  0000000000000000(0000) GS:ffff8880ae600000(0063) knlGS:0000000009cf4840
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000020000080 CR3: 000000009fa0d000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
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
RIP: 0023:0xf7fe9569
Code: 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 002b:00000000ffd99e7c EFLAGS: 00000217 ORIG_RAX: 000000000000001a
RAX: ffffffffffffffda RBX: 000000000000000d RCX: 0000000000001ad1
RDX: 0000000000000000 RSI: 0000000020000080 RDI: 0000000000001ad1
RBP: 000000000000003c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
---[ end trace f79c8042700af7c5 ]---
RIP: 0010:x86_fsgsbase_read_task+0x16d/0x310 arch/x86/kernel/process_64.c:393
Code: 3c 02 00 0f 85 7c 01 00 00 4c 8b a5 80 04 00 00 44 0f b7 eb 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 08 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 30 01 00 00 45 3b 6c 24 08 0f
RSP: 0018:ffffc90001677db8 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: 0000000000000008 RDI: 0000000000000008
RBP: ffff88809eaf9780 R08: 0000000000000000 R09: ffff88809eaf9c17
R10: ffffed1013d5f382 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000020000080
FS:  0000000000000000(0000) GS:ffff8880ae600000(0063) knlGS:0000000009cf4840
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000020000080 CR3: 000000009fa0d000 CR4: 00000000001506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Fixes: 07e1d88adaae ("x86/fsgsbase/64: Fix ptrace() to read the FS/GS base accurately")
Co-developed-by: Jann Horn <jannh@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: 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>
Cc: Ingo Molnar <mingo@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 d6f946707270e41d4aa04e4069b1b4eb82afd687..8393ee1aa9c89b3712b7aef510149d73762ef314 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -390,7 +390,7 @@ 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 == NULL || idx >= ldt->nr_entries))
 			base = 0;
 		else
 			base = get_desc_base(ldt->entries + idx);
-- 
2.28.0.220.ged08abb693-goog

Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Andy Lutomirski]

> On Aug 14, 2020, at 11:16 AM, Eric Dumazet <edumazet@google.com> wrote:
> 
> syzbot found its way in 86_fsgsbase_read_task() [1]
> 
> Fix is to make sure ldt pointer is not NULL

Acked-by: Andy Lutomirski <luto@kernel.org>

Maybe add something like this to the changelog:

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.

I’ll see if I can whip up a test case too.

> 
> [1]
> general protection fault, probably for non-canonical address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN
> KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
> CPU: 0 PID: 6866 Comm: syz-executor262 Not tainted 5.8.0-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:x86_fsgsbase_read_task+0x16d/0x310 arch/x86/kernel/process_64.c:393
> Code: 3c 02 00 0f 85 7c 01 00 00 4c 8b a5 80 04 00 00 44 0f b7 eb 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 08 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 30 01 00 00 45 3b 6c 24 08 0f
> RSP: 0018:ffffc90001677db8 EFLAGS: 00010202
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000000001 RSI: 0000000000000008 RDI: 0000000000000008
> RBP: ffff88809eaf9780 R08: 0000000000000000 R09: ffff88809eaf9c17
> R10: ffffed1013d5f382 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000020000080
> FS:  0000000000000000(0000) GS:ffff8880ae600000(0063) knlGS:0000000009cf4840
> CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
> CR2: 0000000020000080 CR3: 000000009fa0d000 CR4: 00000000001506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 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
> RIP: 0023:0xf7fe9569
> Code: 10 05 03 74 b8 01 10 06 03 74 b4 01 10 07 03 74 b0 01 10 08 03 74 d8 01 00 00 00 00 00 00 00 00 00 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
> RSP: 002b:00000000ffd99e7c EFLAGS: 00000217 ORIG_RAX: 000000000000001a
> RAX: ffffffffffffffda RBX: 000000000000000d RCX: 0000000000001ad1
> RDX: 0000000000000000 RSI: 0000000020000080 RDI: 0000000000001ad1
> RBP: 000000000000003c R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> Modules linked in:
> ---[ end trace f79c8042700af7c5 ]---
> RIP: 0010:x86_fsgsbase_read_task+0x16d/0x310 arch/x86/kernel/process_64.c:393
> Code: 3c 02 00 0f 85 7c 01 00 00 4c 8b a5 80 04 00 00 44 0f b7 eb 48 b8 00 00 00 00 00 fc ff df 49 8d 7c 24 08 48 89 fa 48 c1 ea 03 <0f> b6 04 02 84 c0 74 08 3c 03 0f 8e 30 01 00 00 45 3b 6c 24 08 0f
> RSP: 0018:ffffc90001677db8 EFLAGS: 00010202
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000000001 RSI: 0000000000000008 RDI: 0000000000000008
> RBP: ffff88809eaf9780 R08: 0000000000000000 R09: ffff88809eaf9c17
> R10: ffffed1013d5f382 R11: 0000000000000000 R12: 0000000000000000
> R13: 0000000000000000 R14: 0000000000000000 R15: 0000000020000080
> FS:  0000000000000000(0000) GS:ffff8880ae600000(0063) knlGS:0000000009cf4840
> CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
> CR2: 0000000020000080 CR3: 000000009fa0d000 CR4: 00000000001506f0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 
> Fixes: 07e1d88adaae ("x86/fsgsbase/64: Fix ptrace() to read the FS/GS base accurately")
> Co-developed-by: Jann Horn <jannh@google.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: 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>
> Cc: Ingo Molnar <mingo@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 d6f946707270e41d4aa04e4069b1b4eb82afd687..8393ee1aa9c89b3712b7aef510149d73762ef314 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -390,7 +390,7 @@ 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 == NULL || idx >= ldt->nr_entries))
>            base = 0;
>        else
>            base = get_desc_base(ldt->entries + idx);
> -- 
> 2.28.0.220.ged08abb693-goog
> 

Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Eric Dumazet]

On Fri, Aug 14, 2020 at 12:03 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
>
>
> > On Aug 14, 2020, at 11:16 AM, Eric Dumazet <edumazet@google.com> wrote:
> >
> > syzbot found its way in 86_fsgsbase_read_task() [1]
> >
> > Fix is to make sure ldt pointer is not NULL
>
> Acked-by: Andy Lutomirski <luto@kernel.org>
>
> Maybe add something like this to the changelog:
>
> 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.
>
> I’ll see if I can whip up a test case too.
>

Jann has a repro if needed (and syzbot also had one)

Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Jann Horn]

On Fri, Aug 14, 2020 at 9:03 PM Andy Lutomirski <luto@amacapital.net> wrote:
> > On Aug 14, 2020, at 11:16 AM, Eric Dumazet <edumazet@google.com> wrote:
> >
> > syzbot found its way in 86_fsgsbase_read_task() [1]
> >
> > Fix is to make sure ldt pointer is not NULL
>
> Acked-by: Andy Lutomirski <luto@kernel.org>
>
> Maybe add something like this to the changelog:
>
> 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.
>
> I’ll see if I can whip up a test case too.

This is the reproducer I used to test this on 4.20.17:

#include <stdio.h>
#include <unistd.h>
#include <err.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <sys/user.h>
#include <sys/prctl.h>
#include <signal.h>
#include <stddef.h>
#include <errno.h>

#define SEL_LDT 0x4
#define USER_RPL 0x3
#define LDT_SELECTOR(idx) (((idx)<<3) | SEL_LDT | USER_RPL)

int main(void) {
  pid_t child = fork();
  if (child == -1) err(1, "fork");
  if (child == 0) {
    prctl(PR_SET_PDEATHSIG, SIGKILL);
    while (1) pause();
  }
  if (ptrace(PTRACE_ATTACH, child, NULL, NULL)) err(1, "PTRACE_ATTACH");
  int status;
  if (waitpid(child, &status, __WALL) != child) err(1, "waitpid");
  if (ptrace( PTRACE_POKEUSER, child, (void*)offsetof(struct
user_regs_struct, fs), (void*)LDT_SELECTOR(0) ))
    err(1, "PTRACE_POKEUSER");
  errno = 0;
  unsigned long val = ptrace( PTRACE_PEEKUSER, child,
(void*)offsetof(struct user_regs_struct, fs_base), NULL );
  printf("PTRACE_PEEKUSER returns user_regs_struct.fs_base = 0x%lx
(%m)\n", val);
}

Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Ingo Molnar]

* Eric Dumazet <edumazet@google.com> wrote:

> syzbot found its way in 86_fsgsbase_read_task() [1]
> 
> Fix is to make sure ldt pointer is not NULL.

Thanks for this fix. Linus has picked it up (inclusive the typos to 
the x86_fsgsbase_read_task() function name ;-), it's now upstream 
under:

  8ab49526b53d: ("x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task")

By the fixes tag it looks like this should probably be backported all 
the way back to ~v4.20 or so?

Thanks,

	Ingo

Re: [PATCH] x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

[Eric Dumazet]

On Sat, Aug 15, 2020 at 4:48 AM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Eric Dumazet <edumazet@google.com> wrote:
>
> > syzbot found its way in 86_fsgsbase_read_task() [1]
> >
> > Fix is to make sure ldt pointer is not NULL.
>
> Thanks for this fix. Linus has picked it up (inclusive the typos to
> the x86_fsgsbase_read_task() function name ;-), it's now upstream
> under:
>
>   8ab49526b53d: ("x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task")
>
> By the fixes tag it looks like this should probably be backported all
> the way back to ~v4.20 or so?

This is absolutely right, sorry about the lack of a stable tag.

Most of my patches usually land into David Miller trees, where the
stable tag is not welcomed.
We use  Fixes: tags to convey the exact information needed for stable backports.

Thanks.