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x86: don't use privilege stack areas as a guard

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This is causing problems, as if we create a thread in
a system call we will *not* be using the kernel page
tables if CONFIG_KPTI=n.

Just don't fiddle with this page's permissions; we don't
need it as a guard area anyway since we have a stack
guard placed immediately before it, and this page
is unused if user mode isn't active.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
This commit is contained in:
Andrew Boie 2019-11-21 19:52:11 -08:00 • committed by Anas Nashif
commit 708d5f7922
1 changed files with 0 additions and 29 deletions
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29
arch/x86/core/userspace.c
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@ -87,22 +87,8 @@ FUNC_NORETURN static void drop_to_user(k_thread_entry_t user_entry,
CODE_UNREACHABLE;
}
static inline void
set_privilege_stack_perms(struct z_x86_thread_stack_header *header,
bool is_usermode)
{
/* Set MMU properties for the privilege mode elevation stack. If we're
* not in user mode, this functions as a guard area.
*/
z_x86_mmu_set_flags(&z_x86_kernel_ptables, &header->privilege_stack,
MMU_PAGE_SIZE,
is_usermode ? MMU_ENTRY_WRITE : MMU_ENTRY_READ,
Z_X86_MMU_RW, true);
}
/* Does the following:
*
* - Allows the kernel to write to the privilege elevation stack area.
* - Initialize per-thread page tables and update thread->arch.ptables to
* point to them.
* - Set thread->arch.psp to point to the initial stack pointer for user
@ -117,12 +103,6 @@ static void prepare_user_thread(struct k_thread *thread)
__ASSERT((thread->base.user_options & K_USER) != 0,
"not a user thread");
/* Set privileve elevation stack area to writable. Need to do this
* before calling z_x86_pt_init(), as on 32-bit the top-level PDPT
* is in there as well.
*/
set_privilege_stack_perms(header, true);
/* Create and program into the MMU the per-thread page tables */
z_x86_thread_pt_init(thread);
@ -132,16 +112,7 @@ static void prepare_user_thread(struct k_thread *thread)
static void prepare_supervisor_thread(struct k_thread *thread)
{
struct z_x86_thread_stack_header *header =
(struct z_x86_thread_stack_header *)thread->stack_obj;
thread->arch.ptables = &z_x86_kernel_ptables;
/* Privilege elevation stack set to read-only to function
* as a guard area. This gets made writable if we drop
* to user mode later.
*/
set_privilege_stack_perms(header, false);
}
/* Preparation steps needed for all threads if user mode is turned on.