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SubjectRe: [RFC PATCH] mm: add probe_user_read() and probe_user_address()
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Le 19/10/2018 à 17:42, Kees Cook a écrit :
> On Fri, Oct 19, 2018 at 8:14 AM, Christophe Leroy
> <christophe.leroy@c-s.fr> wrote:
>> In the powerpc, there are several places implementing safe
>> access to user data. This is sometimes implemented using
>> probe_kerne_address() with additional access_ok() verification,
>> sometimes with get_user() enclosed in a pagefault_disable()/enable()
>> pair, etc... :
>> show_user_instructions()
>> bad_stack_expansion()
>> p9_hmi_special_emu()
>> fsl_pci_mcheck_exception()
>> read_user_stack_64()
>> read_user_stack_32() on PPC64
>> read_user_stack_32() on PPC32
>> power_pmu_bhrb_to()
>>
>> In the same spirit as probe_kernel_read() and probe_kernel_address(),
>> this patch adds probe_user_read() and probe_user_address().
>>
>> probe_user_read() does the same as probe_kernel_read() but
>> first checks that it is really a user address.
>>
>> probe_user_address() is a shortcut to probe_user_read()
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>> ---
>> include/linux/uaccess.h | 10 ++++++++++
>> mm/maccess.c | 33 +++++++++++++++++++++++++++++++++
>> 2 files changed, 43 insertions(+)
>>
>> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
>> index efe79c1cdd47..fb00e3f847d7 100644
>> --- a/include/linux/uaccess.h
>> +++ b/include/linux/uaccess.h
>> @@ -266,6 +266,16 @@ extern long strncpy_from_unsafe(char *dst, const void *unsafe_addr, long count);
>> #define probe_kernel_address(addr, retval) \
>> probe_kernel_read(&retval, addr, sizeof(retval))
>>
>> +/**
>> + * probe_user_address(): safely attempt to read from a user location
>> + * @addr: address to read from
>> + * @retval: read into this variable
>> + *
>> + * Returns 0 on success, or -EFAULT.
>> + */
>> +#define probe_user_address(addr, retval) \
>> + probe_user_read(&(retval), addr, sizeof(retval))
>> +
>> #ifndef user_access_begin
>> #define user_access_begin() do { } while (0)
>> #define user_access_end() do { } while (0)
>> diff --git a/mm/maccess.c b/mm/maccess.c
>> index ec00be51a24f..85d4a88a6917 100644
>> --- a/mm/maccess.c
>> +++ b/mm/maccess.c
>> @@ -67,6 +67,39 @@ long __probe_kernel_write(void *dst, const void *src, size_t size)
>> EXPORT_SYMBOL_GPL(probe_kernel_write);
>>
>> /**
>> + * probe_user_read(): safely attempt to read from a user location
>> + * @dst: pointer to the buffer that shall take the data
>> + * @src: address to read from
>> + * @size: size of the data chunk
>> + *
>> + * Safely read from address @src to the buffer at @dst. If a kernel fault
>> + * happens, handle that and return -EFAULT.
>> + *
>> + * We ensure that the copy_from_user is executed in atomic context so that
>> + * do_page_fault() doesn't attempt to take mmap_sem. This makes
>> + * probe_user_read() suitable for use within regions where the caller
>> + * already holds mmap_sem, or other locks which nest inside mmap_sem.
>> + */
>> +
>> +long __weak probe_user_read(void *dst, const void *src, size_t size)
>> + __attribute__((alias("__probe_user_read")));
>
> Let's use #defines to deal with per-arch aliases so we can keep the
> inline I'm suggesting below...
>
>> +
>> +long __probe_user_read(void *dst, const void __user *src, size_t size)
>
> Please make this __always_inline so the "size" variable can be
> examined for const-ness by the check_object_size() in
> __copy_from_user_inatomic().

Ok, I did as suggested in the patch I just sent.

Would it be worth doing the same with the existing probe_kernel_read()
and probe_kernel_write() ?

Christophe

>
> -Kees
>

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