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SubjectRE: [PATCH v6 0/3] Harden spectrev2 userspace-userspace protection
Date
> -----Original Message-----
> From: Jiri Kosina [mailto:jikos@kernel.org]
> Sent: Wednesday, September 12, 2018 2:05 AM
> To: Thomas Gleixner <tglx@linutronix.de>; Ingo Molnar <mingo@redhat.com>;
> Peter Zijlstra <peterz@infradead.org>; Josh Poimboeuf
> <jpoimboe@redhat.com>; Andrea Arcangeli <aarcange@redhat.com>;
> Woodhouse, David <dwmw@amazon.co.uk>; Andi Kleen <ak@linux.intel.com>;
> Tim Chen <tim.c.chen@linux.intel.com>; Schaufler, Casey
> <casey.schaufler@intel.com>
> Cc: linux-kernel@vger.kernel.org; x86@kernel.org
> Subject: [PATCH v6 0/3] Harden spectrev2 userspace-userspace protection
>
> Currently, linux kernel is basically not preventing userspace-userspace
> spectrev2 attack, because:
>
> - IBPB is basically unused (issued only for tasks that marked themselves
> explicitly non-dumpable, which is absolutely negligible minority of all
> software out there), therefore cross-process branch buffer posioning
> using spectrev2 is possible
>
> - STIBP is completely unused, therefore cross-process branch buffer
> poisoning using spectrev2 between processess running on two HT siblings
> thread s is possible
>
> This patchset changes IBPB semantics, so that it's now applied whenever
> context-switching between processess that can't use ptrace() to achieve
> the same. This admittedly comes with extra overhad on a context switch;
> systems that don't care about could disable the mitigation using
> nospectre_v2 boot option.
> The IBPB implementaion is heavily based on original patches by Tim Chen.
>
> In addition to that, we unconditionally turn STIBP on so that HT siblings
> always have separate branch buffers.
>
> We've been carrying IBPB implementation with the same semantics in our
> (SUSE) trees since january disclosure; STIBP was more or less ignored up
> to today.
>
> v1->v2:
> include IBPB changes
> v2->v3:
> fix IBPB 'who can trace who' semantics
> wire up STIBP flipping to SMT hotplug
> v3->v4:
> dropped ___ptrace_may_access(), as it's not needed
> fixed deadlock with LSM/audit/selinux (Andrea Arcangeli)
> statically patch out the ptrace check if !IBPB
>
> v4->v5:
> fix MSR writing logic (Thomas Gleixner, Josh Poimboeuf)
>
> v5->v6:
> propagate X86_FEATURE_RSB_CTXSW setting to sysfs
> propagate STIBP setting to sysfs (Thomas Gleixner)
> simplify arch_smt_update() (Thomas Gleixner)
>
> Jiri Kosina (3):
> x86/speculation: apply IBPB more strictly to avoid cross-process data leak
> x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation
> x86/speculation: Propagate information about RSB filling mitigation to sysfs
>
> arch/x86/kernel/cpu/bugs.c | 60
> ++++++++++++++++++++++++++++++++++++++++++++++++++++++------
> arch/x86/mm/tlb.c | 31 ++++++++++++++++++++-----------
> include/linux/ptrace.h | 4 ++++
> kernel/cpu.c | 11 ++++++++++-
> kernel/ptrace.c | 12 ++++++++----
> 5 files changed, 96 insertions(+), 22 deletions(-)
>
> --
> Jiri Kosina
> SUSE Labs

The locking issue with SELinux has a simple fix as below.
The other LSMs don't manifest this issue. With the change to
SELinux the call to security_ptrace_access_check() can and
should be made unconditionally.

Patch is attached, whitespace damaged (known problem) patch:

SELinux: Handle audit locking for PTRACE_MODE_IBPB

The SELinux audit code locking cannot be used from the
task switching code, which is where PTRACE_MODE_IBPB comes
from. As this is a system check, not a user action, audit
is not needed, and would generate noise. Use the unaudited
check for this case.

Signed-off-by: Casey Schaufler <casey.schaufler@intel.com>
---
kernel/ptrace.c | 4 +---
security/selinux/hooks.c | 5 +++++
2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 5c5e7cb597cd..202a4d9c2af7 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -330,9 +330,7 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
!ptrace_has_cap(mm->user_ns, mode))))
return -EPERM;

- if (!(mode & PTRACE_MODE_NOACCESS_CHK))
- return security_ptrace_access_check(task, mode);
- return 0;
+ return security_ptrace_access_check(task, mode);
}

bool ptrace_may_access(struct task_struct *task, unsigned int mode)
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 161a4f29f860..30d21142e9fe 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2215,7 +2215,12 @@ static int selinux_ptrace_access_check(struct task_struct *child,
{
u32 sid = current_sid();
u32 csid = task_sid(child);
+ struct av_decision avd;

+ if (mode == PTRACE_MODE_IBPB)
+ return avc_has_perm_noaudit(&selinux_state, sid, csid,
+ SECCLASS_PROCESS, PROCESS__PTRACE,
+ 0, &avd);
if (mode & PTRACE_MODE_READ)
return avc_has_perm(&selinux_state,
sid, csid, SECCLASS_FILE, FILE__READ, NULL);
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