[PATCH v2] x86/vm86/32: Remove VM86_SCREEN_BITMAP support
From: Andy Lutomirski
Date: Tue Jan 19 2021 - 13:55:18 EST
The implementation was rather buggy. It unconditionally marked PTEs
read-only, even for VM_SHARED mappings. I'm not sure whether this is
actually a problem, but it certainly seems unwise. More importantly, it
released the mmap lock before flushing the TLB, which could allow a racing
CoW operation to falsely believe that the underlying memory was not
writable.
I can't find any users at all of this mechanism, so just remove it.
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Linux-MM <linux-mm@xxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Stas Sergeev <stsp@xxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
Changes from v1:
- Get rid of the whole screen_bitmap and the fault code, too.
arch/x86/include/asm/vm86.h | 1 -
arch/x86/include/uapi/asm/vm86.h | 4 +--
arch/x86/kernel/vm86_32.c | 62 ++++++++------------------------
arch/x86/mm/fault.c | 30 ----------------
4 files changed, 16 insertions(+), 81 deletions(-)
diff --git a/arch/x86/include/asm/vm86.h b/arch/x86/include/asm/vm86.h
index 26efbec94448..9e8ac5073ecb 100644
--- a/arch/x86/include/asm/vm86.h
+++ b/arch/x86/include/asm/vm86.h
@@ -36,7 +36,6 @@ struct vm86 {
unsigned long saved_sp0;
unsigned long flags;
- unsigned long screen_bitmap;
unsigned long cpu_type;
struct revectored_struct int_revectored;
struct revectored_struct int21_revectored;
diff --git a/arch/x86/include/uapi/asm/vm86.h b/arch/x86/include/uapi/asm/vm86.h
index d2ee4e307ef8..18909b8050bc 100644
--- a/arch/x86/include/uapi/asm/vm86.h
+++ b/arch/x86/include/uapi/asm/vm86.h
@@ -97,7 +97,7 @@ struct revectored_struct {
struct vm86_struct {
struct vm86_regs regs;
unsigned long flags;
- unsigned long screen_bitmap;
+ unsigned long screen_bitmap; /* unused, preserved by vm86() */
unsigned long cpu_type;
struct revectored_struct int_revectored;
struct revectored_struct int21_revectored;
@@ -106,7 +106,7 @@ struct vm86_struct {
/*
* flags masks
*/
-#define VM86_SCREEN_BITMAP 0x0001
+#define VM86_SCREEN_BITMAP 0x0001 /* no longer supported */
struct vm86plus_info_struct {
unsigned long force_return_for_pic:1;
diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index 764573de3996..e5a7a10a0164 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -134,7 +134,11 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
unsafe_put_user(regs->ds, &user->regs.ds, Efault_end);
unsafe_put_user(regs->fs, &user->regs.fs, Efault_end);
unsafe_put_user(regs->gs, &user->regs.gs, Efault_end);
- unsafe_put_user(vm86->screen_bitmap, &user->screen_bitmap, Efault_end);
+
+ /*
+ * Don't write screen_bitmap in case some user had a value there
+ * and expected it to remain unchanged.
+ */
user_access_end();
@@ -160,49 +164,6 @@ void save_v86_state(struct kernel_vm86_regs *regs, int retval)
do_exit(SIGSEGV);
}
-static void mark_screen_rdonly(struct mm_struct *mm)
-{
- struct vm_area_struct *vma;
- spinlock_t *ptl;
- pgd_t *pgd;
- p4d_t *p4d;
- pud_t *pud;
- pmd_t *pmd;
- pte_t *pte;
- int i;
-
- mmap_write_lock(mm);
- pgd = pgd_offset(mm, 0xA0000);
- if (pgd_none_or_clear_bad(pgd))
- goto out;
- p4d = p4d_offset(pgd, 0xA0000);
- if (p4d_none_or_clear_bad(p4d))
- goto out;
- pud = pud_offset(p4d, 0xA0000);
- if (pud_none_or_clear_bad(pud))
- goto out;
- pmd = pmd_offset(pud, 0xA0000);
-
- if (pmd_trans_huge(*pmd)) {
- vma = find_vma(mm, 0xA0000);
- split_huge_pmd(vma, pmd, 0xA0000);
- }
- if (pmd_none_or_clear_bad(pmd))
- goto out;
- pte = pte_offset_map_lock(mm, pmd, 0xA0000, &ptl);
- for (i = 0; i < 32; i++) {
- if (pte_present(*pte))
- set_pte(pte, pte_wrprotect(*pte));
- pte++;
- }
- pte_unmap_unlock(pte, ptl);
-out:
- mmap_write_unlock(mm);
- flush_tlb_mm_range(mm, 0xA0000, 0xA0000 + 32*PAGE_SIZE, PAGE_SHIFT, false);
-}
-
-
-
static int do_vm86_irq_handling(int subfunction, int irqnumber);
static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus);
@@ -282,6 +243,15 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
offsetof(struct vm86_struct, int_revectored)))
return -EFAULT;
+
+ /* VM86_SCREEN_BITMAP had numerous bugs and appears to have no users. */
+ if (v.flags & VM86_SCREEN_BITMAP) {
+ char comm[TASK_COMM_LEN];
+
+ pr_info_once("vm86: '%s' uses VM86_SCREEN_BITMAP, which is no longer supported\n", get_task_comm(comm, current));
+ return -EINVAL;
+ }
+
memset(&vm86regs, 0, sizeof(vm86regs));
vm86regs.pt.bx = v.regs.ebx;
@@ -302,7 +272,6 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
vm86regs.gs = v.regs.gs;
vm86->flags = v.flags;
- vm86->screen_bitmap = v.screen_bitmap;
vm86->cpu_type = v.cpu_type;
if (copy_from_user(&vm86->int_revectored,
@@ -370,9 +339,6 @@ static long do_sys_vm86(struct vm86plus_struct __user *user_vm86, bool plus)
update_task_stack(tsk);
preempt_enable();
- if (vm86->flags & VM86_SCREEN_BITMAP)
- mark_screen_rdonly(tsk->mm);
-
memcpy((struct kernel_vm86_regs *)regs, &vm86regs, sizeof(vm86regs));
return regs->ax;
}
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 82bf37a5c9ec..fd0dd60f6302 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -261,25 +261,6 @@ void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
}
}
-/*
- * Did it hit the DOS screen memory VA from vm86 mode?
- */
-static inline void
-check_v8086_mode(struct pt_regs *regs, unsigned long address,
- struct task_struct *tsk)
-{
-#ifdef CONFIG_VM86
- unsigned long bit;
-
- if (!v8086_mode(regs) || !tsk->thread.vm86)
- return;
-
- bit = (address - 0xA0000) >> PAGE_SHIFT;
- if (bit < 32)
- tsk->thread.vm86->screen_bitmap |= 1 << bit;
-#endif
-}
-
static bool low_pfn(unsigned long pfn)
{
return pfn < max_low_pfn;
@@ -334,15 +315,6 @@ KERN_ERR
"******* Disabling USB legacy in the BIOS may also help.\n";
#endif
-/*
- * No vm86 mode in 64-bit mode:
- */
-static inline void
-check_v8086_mode(struct pt_regs *regs, unsigned long address,
- struct task_struct *tsk)
-{
-}
-
static int bad_address(void *p)
{
unsigned long dummy;
@@ -1395,8 +1367,6 @@ void do_user_addr_fault(struct pt_regs *regs,
mm_fault_error(regs, hw_error_code, address, fault);
return;
}
-
- check_v8086_mode(regs, address, tsk);
}
NOKPROBE_SYMBOL(do_user_addr_fault);
--
2.29.2