[PATCH 2/2] mm: make copy_to_kernel_nofault() not fault on user addresses

[Christophe Leroy]

Hi,

Le 02/09/2024 à 07:31, Omar Sandoval a écrit :
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> 
> From: Omar Sandoval <osandov at fb.com>
> 
> I found that on x86, copy_to_kernel_nofault() still faults on addresses
> outside of the kernel address range (including NULL):
> 
>    # echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
>    # echo g > /proc/sysrq-trigger
>    ...
>    [15]kdb> mm 0 1234
>    [   94.652476] BUG: kernel NULL pointer dereference, address: 0000000000000000
...
> 
> Note that copy_to_kernel_nofault() uses pagefault_disable(), but it
> still faults. This is because with Supervisor Mode Access Prevention
> (SMAP) enabled, do_user_addr_fault() Oopses on a fault for a user
> address from kernel space _before_ checking faulthandler_disabled().
> 
> copy_from_kernel_nofault() avoids this by checking that the address is
> in the kernel before doing the actual memory access. Do the same in
> copy_to_kernel_nofault() so that we get an error as expected:
> 
>    # echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
>    # echo g > /proc/sysrq-trigger
>    ...
>    [17]kdb> mm 0 1234
>    kdb_putarea_size: Bad address 0x0
>    diag: -21: Invalid address
> 
> Signed-off-by: Omar Sandoval <osandov at fb.com>
> ---
>   mm/maccess.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/mm/maccess.c b/mm/maccess.c
> index 72e9c03ea37f..d67dee51a1cc 100644
> --- a/mm/maccess.c
> +++ b/mm/maccess.c
> @@ -61,6 +61,9 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
>          if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
>                  align = (unsigned long)dst | (unsigned long)src;
> 
> +       if (!copy_kernel_nofault_allowed(dst, size))
> +               return -ERANGE;
> +
>          pagefault_disable();
>          if (!(align & 7))
>                  copy_to_kernel_nofault_loop(dst, src, size, u64, Efault);
> --
> 2.46.0
> 

This patch leads to the following errors on ppc64le_defconfig:

[    2.423930][    T1] Running code patching self-tests ...
[    2.428912][    T1] code-patching: test failed at line 395
[    2.429085][    T1] code-patching: test failed at line 398
[    2.429561][    T1] code-patching: test failed at line 432
[    2.429679][    T1] code-patching: test failed at line 435

This seems to be linked to commit c28c15b6d28a ("powerpc/code-patching: 
Use temporary mm for Radix MMU"), copy_from_kernel_nofault_allowed() 
returns false for the patching area.

Christophe