From: Yu-cheng Yu <redacted>

When a process is duplicated, but the child shares the address space with
the parent, there is potential for the threads sharing a single stack to
cause conflicts for each other. In the normal non-cet case this is handled
in two ways.

With regular CLONE_VM a new stack is provided by userspace such that the
parent and child have different stacks.

For vfork, the parent is suspended until the child exits. So as long as
the child doesn't return from the vfork()/CLONE_VFORK calling function and
sticks to a limited set of operations, the parent and child can share the
same stack.

For shadow stack, these scenarios present similar sharing problems. For the
CLONE_VM case, the child and the parent must have separate shadow stacks.
Instead of changing clone to take a shadow stack, have the kernel just
allocate one and switch to it.

Use stack_size passed from clone3() syscall for thread shadow stack size. A
compat-mode thread shadow stack size is further reduced to 1/4. This
allows more threads to run in a 32-bit address space. The clone() does not
pass stack_size, which was added to clone3(). In that case, use
RLIMIT_STACK size and cap to 4 GB.

For shadow stack enabled vfork(), the parent and child can share the same
shadow stack, like they can share a normal stack. Since the parent is
suspended until the child terminates, the child will not interfere with
the parent while executing as long as it doesn't return from the vfork()
and overwrite up the shadow stack. The child can safely overwrite down
the shadow stack, as the parent can just overwrite this later. So CET does
not add any additional limitations for vfork().

Userspace implementing posix vfork() can actually prevent the child from
returning from the vfork() calling function, using CET. Glibc does this
by adjusting the shadow stack pointer in the child, so that the child
receives a #CP if it tries to return from vfork() calling function.

Free the shadow stack on thread exit by doing it in mm_release(). Skip
this when exiting a vfork() child since the stack is shared in the
parent.

During this operation, the shadow stack pointer of the new thread needs
to be updated to point to the newly allocated shadow stack. Since the
ability to do this is confined to the FPU subsystem, change
fpu_clone() to take the new shadow stack pointer, and update it
internally inside the FPU subsystem. This part was suggested by Thomas
Gleixner.

Suggested-by: Thomas Gleixner <redacted>
Signed-off-by: Yu-cheng Yu <redacted>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

---

v2:
 - Have fpu_clone() take new shadow stack pointer and update SSP in
   xsave buffer for new task. (tglx)

v1:
 - Expand commit log.
 - Add more comments.
 - Switch to xsave helpers.

Yu-cheng v30:
 - Update comments about clone()/clone3(). (Borislav Petkov)

Yu-cheng v29:
 - WARN_ON_ONCE() when get_xsave_addr() returns NULL, and update comments.
   (Dave Hansen)

 arch/x86/include/asm/cet.h         |  7 +++++
 arch/x86/include/asm/fpu/sched.h   |  3 +-
 arch/x86/include/asm/mmu_context.h |  2 ++
 arch/x86/kernel/fpu/core.c         | 40 ++++++++++++++++++++++++-
 arch/x86/kernel/process.c          | 17 ++++++++++-
 arch/x86/kernel/shstk.c            | 48 +++++++++++++++++++++++++++++-
 6 files changed, 113 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 778d3054ccc7..f332e9b42b6d 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c

@@ -555,8 +555,40 @@ static inline void fpu_inherit_perms(struct fpu *dst_fpu)
 	}
 }
 
+#ifdef CONFIG_X86_SHADOW_STACK
+static int update_fpu_shstk(struct task_struct *dst, unsigned long ssp)
+{
+	struct cet_user_state *xstate;
+
+	/* If ssp update is not needed. */
+	if (!ssp)
+		return 0;
+
+	xstate = get_xsave_addr(&dst->thread.fpu.fpstate->regs.xsave,
+				XFEATURE_CET_USER);
+
+	/*
+	 * If there is a non-zero ssp, then 'dst' must be configured with a shadow
+	 * stack and the fpu state should be up to date since it was just copied
+	 * from the parent in fpu_clone(). So there must be a valid non-init CET
+	 * state location in the buffer.
+	 */
+	if (WARN_ON_ONCE(!xstate))
+		return 1;
+
+	xstate->user_ssp = (u64)ssp;
+
+	return 0;
+}
+#else
+static int update_fpu_shstk(struct task_struct *dst, unsigned long shstk_addr)
+{