Re: [Xen-devel] [PATCH MM-PART3 v3 3/9] xen/arm: mm: Sanity check any update of Xen page tables

[Stefano Stabellini <sstabellini@kernel.org>]

On Fri, 14 Jun 2019, Julien Grall wrote:
> The code handling Xen PT update has quite a few restrictions on what it
> can do. This is not a bad thing as it keeps the code simple.
> 
> There are already a few checks scattered in current page table handling.
> However they are not sufficient as they could still allow to
> modify/remove entry with contiguous bit set.
> 
> The checks are divided in two sets:
>     - per entry check: They are gathered in a new function that will
>     check whether an update is valid based on the flags passed and the
>     current value of an entry.
>     - global check: They are sanity check on xen_pt_update() parameters.
> 
> Additionally to contiguous check, we also now check that the caller is
> not trying to modify the memory attributes of an entry.
> 
> Lastly, it was probably a bit over the top to forbid removing an
> invalid mapping. This could just be ignored. The new behavior will be
> helpful in future changes.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> Reviewed-by: Andrii Anisov <andrii_anisov@epam.com>

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> ---
>     Changes in v3:
>         - Only allow modification on valid entry
> 
>     Changes in v2:
>         - Correctly detect the removal of a page
>         - Fix ASSERT on flags in the else case
>         - Add Andrii's reviewed-by
> ---
>  xen/arch/arm/mm.c | 122 ++++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 104 insertions(+), 18 deletions(-)
> 
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index b13d9adf40..dcf041578b 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -50,6 +50,19 @@
>  #undef mfn_to_virt
>  #define mfn_to_virt(mfn) __mfn_to_virt(mfn_x(mfn))
>  
> +#ifdef NDEBUG
> +static inline void
> +__attribute__ ((__format__ (__printf__, 1, 2)))
> +mm_printk(const char *fmt, ...) {}
> +#else
> +#define mm_printk(fmt, args...)             \
> +    do                                      \
> +    {                                       \
> +        dprintk(XENLOG_ERR, fmt, ## args);  \
> +        WARN();                             \
> +    } while (0);
> +#endif
> +
>  #define DEFINE_PAGE_TABLES(name, nr)                    \
>  lpae_t __aligned(PAGE_SIZE) name[LPAE_ENTRIES * (nr)]
>  
> @@ -941,12 +954,81 @@ enum xenmap_operation {
>      RESERVE
>  };
>  
> +/* Sanity check of the entry */
> +static bool xen_pt_check_entry(lpae_t entry, mfn_t mfn, unsigned int flags)
> +{
> +    /* Sanity check when modifying a page. */
> +    if ( (flags & _PAGE_PRESENT) && mfn_eq(mfn, INVALID_MFN) )
> +    {
> +        /* We don't allow modifying an invalid entry. */
> +        if ( !lpae_is_valid(entry) )
> +        {
> +            mm_printk("Modifying invalid entry is not allowed.\n");
> +            return false;
> +        }
> +
> +        /* We don't allow changing memory attributes. */
> +        if ( entry.pt.ai != PAGE_AI_MASK(flags) )
> +        {
> +            mm_printk("Modifying memory attributes is not allowed (0x%x -> 0x%x).\n",
> +                      entry.pt.ai, PAGE_AI_MASK(flags));
> +            return false;
> +        }
> +
> +        /* We don't allow modifying entry with contiguous bit set. */
> +        if ( entry.pt.contig )
> +        {
> +            mm_printk("Modifying entry with contiguous bit set is not allowed.\n");
> +            return false;
> +        }
> +    }
> +    /* Sanity check when inserting a page */
> +    else if ( flags & _PAGE_PRESENT )
> +    {
> +        /* We should be here with a valid MFN. */
> +        ASSERT(!mfn_eq(mfn, INVALID_MFN));
> +
> +        /* We don't allow replacing any valid entry. */
> +        if ( lpae_is_valid(entry) )
> +        {
> +            mm_printk("Changing MFN for a valid entry is not allowed (%#"PRI_mfn" -> %#"PRI_mfn").\n",
> +                      mfn_x(lpae_get_mfn(entry)), mfn_x(mfn));
> +            return false;
> +        }
> +    }
> +    /* Sanity check when removing a page. */
> +    else if ( (flags & (_PAGE_PRESENT|_PAGE_POPULATE)) == 0 )
> +    {
> +        /* We should be here with an invalid MFN. */
> +        ASSERT(mfn_eq(mfn, INVALID_MFN));
> +
> +        /* We don't allow removing page with contiguous bit set. */
> +        if ( entry.pt.contig )
> +        {
> +            mm_printk("Removing entry with contiguous bit set is not allowed.\n");
> +            return false;
> +        }
> +    }
> +    /* Sanity check when populating the page-table. No check so far. */
> +    else
> +    {
> +        ASSERT(flags & _PAGE_POPULATE);
> +        /* We should be here with an invalid MFN */
> +        ASSERT(mfn_eq(mfn, INVALID_MFN));
> +    }
> +
> +    return true;
> +}
> +
>  static int xen_pt_update_entry(enum xenmap_operation op, unsigned long addr,
>                                 mfn_t mfn, unsigned int flags)
>  {
>      lpae_t pte, *entry;
>      lpae_t *third = NULL;
>  
> +    /* _PAGE_POPULATE and _PAGE_PRESENT should never be set together. */
> +    ASSERT((flags & (_PAGE_POPULATE|_PAGE_PRESENT)) != (_PAGE_POPULATE|_PAGE_PRESENT));
> +
>      entry = &xen_second[second_linear_offset(addr)];
>      if ( !lpae_is_valid(*entry) || !lpae_is_table(*entry, 2) )
>      {
> @@ -962,15 +1044,12 @@ static int xen_pt_update_entry(enum xenmap_operation op, unsigned long addr,
>      third = mfn_to_virt(lpae_get_mfn(*entry));
>      entry = &third[third_table_offset(addr)];
>  
> +    if ( !xen_pt_check_entry(*entry, mfn, flags) )
> +        return -EINVAL;
> +
>      switch ( op ) {
>          case INSERT:
>          case RESERVE:
> -            if ( lpae_is_valid(*entry) )
> -            {
> -                printk("%s: trying to replace an existing mapping addr=%lx mfn=%"PRI_mfn"\n",
> -                       __func__, addr, mfn_x(mfn));
> -                return -EINVAL;
> -            }
>              if ( op == RESERVE )
>                  break;
>              pte = mfn_to_xen_entry(mfn, PAGE_AI_MASK(flags));
> @@ -982,12 +1061,6 @@ static int xen_pt_update_entry(enum xenmap_operation op, unsigned long addr,
>              break;
>          case MODIFY:
>          case REMOVE:
> -            if ( !lpae_is_valid(*entry) )
> -            {
> -                printk("%s: trying to %s a non-existing mapping addr=%lx\n",
> -                       __func__, op == REMOVE ? "remove" : "modify", addr);
> -                return -EINVAL;
> -            }
>              if ( op == REMOVE )
>                  pte.bits = 0;
>              else
> @@ -995,12 +1068,6 @@ static int xen_pt_update_entry(enum xenmap_operation op, unsigned long addr,
>                  pte = *entry;
>                  pte.pt.ro = PAGE_RO_MASK(flags);
>                  pte.pt.xn = PAGE_XN_MASK(flags);
> -                if ( !pte.pt.ro && !pte.pt.xn )
> -                {
> -                    printk("%s: Incorrect combination for addr=%lx\n",
> -                           __func__, addr);
> -                    return -EINVAL;
> -                }
>              }
>              write_pte(entry, pte);
>              break;
> @@ -1022,6 +1089,25 @@ static int xen_pt_update(enum xenmap_operation op,
>      int rc = 0;
>      unsigned long addr = virt, addr_end = addr + nr_mfns * PAGE_SIZE;
>  
> +    /*
> +     * The hardware was configured to forbid mapping both writeable and
> +     * executable.
> +     * When modifying/creating mapping (i.e _PAGE_PRESENT is set),
> +     * prevent any update if this happen.
> +     */
> +    if ( (flags & _PAGE_PRESENT) && !PAGE_RO_MASK(flags) &&
> +         !PAGE_XN_MASK(flags) )
> +    {
> +        mm_printk("Mappings should not be both Writeable and Executable.\n");
> +        return -EINVAL;
> +    }
> +
> +    if ( !IS_ALIGNED(virt, PAGE_SIZE) )
> +    {
> +        mm_printk("The virtual address is not aligned to the page-size.\n");
> +        return -EINVAL;
> +    }
> +
>      spin_lock(&xen_pt_lock);
>  
>      for( ; addr < addr_end; addr += PAGE_SIZE )
> -- 
> 2.11.0
> 

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