Re: [PATCH 1/2] arm64: trans_pgd: fix incorrect use of pmd_populate_kernel in copy_pte()

[Rongwei Wang]

On 10/31/21 8:25 PM, Ard Biesheuvel wrote:
> On Sat, 30 Oct 2021 at 20:32, Rongwei Wang
> <rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
> > In commit 5de59884ac0e ("arm64: trans_pgd: pass NULL instead
> > of init_mm to *_populate functions"), simply replace init_mm
> > with NULL for pmd_populate_kernel. But in commit 59511cfd08f3
> > ("arm64: mm: use XN table mapping attributes for user/kernel
> > mappings"), adding the check of mm context in
> > pmd_populate_kernel. And these changes will cause a crash when
> > executing copy_pte/trans_pgd.c, as follows:
> >
> > kernel BUG at arch/arm64/include/asm/pgalloc.h:79!
> > Internal error: Oops - BUG: 0 [#1] SMP
> > Modules linked in: rfkill(E) aes_ce_blk(E) aes_ce_cipher(E) ...
> > CPU: 21 PID: 1617 Comm: a.out Kdump: loaded Tainted: ... 5.15.0-rc7-mm1+ #8
> > Hardware name: ECS, BIOS 0.0.0 02/06/2015
> > pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > pc : trans_pgd_create_copy+0x4ac/0x4f0
> > lr : trans_pgd_create_copy+0x34c/0x4f0
> > sp : ffff80001bf2bc50
> > x29: ffff80001bf2bc50 x28: ffff0010067f1000 x27: ffff800011072000
> > x26: ffff001fffff8000 x25: ffff008000000000 x24: 0040000000000041
> > x23: 0040000000000001 x22: ffff80001bf2bd68 x21: ffff80001188ded8
> > x20: ffff800000000000 x19: ffff000000000000 x18: 0000000000000000
> > x17: 0000000000000000 x16: 0000000000000000 x15: 00000000200004c0
> > x14: ffff00003fffffff x13: ffff007fffffffff x12: ffff800010f882a8
> > x11: 0000000000face57 x10: 0000000000000001 x9 : 0000000000000000
> > x8 : ffff00100cece000 x7 : ffff001001c9f000 x6 : ffff00100ae40000
> > x5 : 0000000000000040 x4 : 0000000000000000 x3 : ffff001fffff7000
> > x2 : ffff000000200000 x1 : ffff000040000000 x0 : ffff00100cecd000
> > Call trace:
> >   trans_pgd_create_copy+0x4ac/0x4f0
> >   machine_kexec_post_load+0x94/0x3bc
> >   do_kexec_load+0x11c/0x2e0
> >   __arm64_sys_kexec_load+0xa8/0xf4
> >   invoke_syscall+0x50/0x120
> >   el0_svc_common.constprop.0+0x58/0x190
> >   do_el0_svc+0x2c/0x90
> >   el0_svc+0x28/0xe0
> >   el0t_64_sync_handler+0xb0/0xb4
> >   el0t_64_sync+0x180/0x184
> > Code: f90000c0 d5033a9f d5033fdf 17ffff7b (d4210000)
> > ---[ end trace cc5461ffe1a085db ]---
> > Kernel panic - not syncing: Oops - BUG: Fatal exception
> >
> > This bug can be reproduced by a user case:
> >
> > void execute_kexec_load(void)
> > {
> >          syscall(__NR_mmap, 0x1ffff000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> >          syscall(__NR_mmap, 0x20000000ul, 0x1000000ul, 7ul, 0x32ul, -1, 0ul);
> >          syscall(__NR_mmap, 0x21000000ul, 0x1000ul, 0ul, 0x32ul, -1, 0ul);
> >
> >          *(uint64_t*)0x200004c0 = 0;
> >          *(uint64_t*)0x200004c8 = 0;
> >          *(uint64_t*)0x200004d0 = 0;
> >          *(uint64_t*)0x200004d8 = 0;
> >          syscall(__NR_kexec_load, 0ul, 1ul, 0x200004c0ul, 0ul);
> > }
> >
> > And this patch just make some simple changes, and including
> > replace pmd_populate_kernel with pmd_populate.
> >
> > Fixes: 59511cfd08f3 ("arm64: mm: use XN table mapping attributes for user/kernel mappings")
> > Reported-by: Abaci <abaci@xxxxxxxxxxxxxxxxx>
> > Signed-off-by: Rongwei Wang <rongwei.wang@xxxxxxxxxxxxxxxxx>
> > ---
> >   arch/arm64/mm/trans_pgd.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/mm/trans_pgd.c b/arch/arm64/mm/trans_pgd.c
> > index d7da8ca40d2e..3f1fc6cb9c9d 100644
> > --- a/arch/arm64/mm/trans_pgd.c
> > +++ b/arch/arm64/mm/trans_pgd.c
> > @@ -62,12 +62,13 @@ static int copy_pte(struct trans_pgd_info *info, pmd_t *dst_pmdp,
> >   {
> >          pte_t *src_ptep;
> >          pte_t *dst_ptep;
> > +       struct page *page;
> >          unsigned long addr = start;
> >
> > -       dst_ptep = trans_alloc(info);
> > -       if (!dst_ptep)
> > +       page = virt_to_page(trans_alloc(info));
> > +       if (!page)
> >                  return -ENOMEM;
> > -       pmd_populate_kernel(NULL, dst_pmdp, dst_ptep);
> > +       pmd_populate(NULL, dst_pmdp, page);
>
> Are you sure this truly fixes the underlying issue rather than the symptom?
Hi Ard

I just found bug line on 'VM_BUG_ON(mm != &init_mm)' shown below, seems 
a obvious incorrect use of 'pmd_populate_kernel' in copy_pte. It seems
these changes were introduced in this year.

static inline void
pmd_populate_kernel(struct mm_struct *mm, pmd_t *pmdp, pte_t *ptep)
{
	VM_BUG_ON(mm != &init_mm);
 	__pmd_populate(pmdp, __pa(ptep), PMD_TYPE_TABLE | PMD_TABLE_UXN);
}

And I had run some testcases, not triggered this bug any more.
If I missing something, please remind me! And I will check it
again.

Thanks!

> pmd_populate() will create a table entry with the PXN attribute set,
> which means nothing below it will be executable by the kernel,
> regardless of the executable permissions at the PTE level.
>
>
> >          dst_ptep = pte_offset_kernel(dst_pmdp, start);
> >
> >          src_ptep = pte_offset_kernel(src_pmdp, start);
> > --
> > 2.27.0