[PATCH v6 08/14] aarch64: enable BTI at runtime

[Richard Henderson]

On 7/13/20 6:28 AM, Szabolcs Nagy wrote:
> The 07/13/2020 14:14, Szabolcs Nagy wrote:
>> The 07/11/2020 08:58, Richard Henderson wrote:
>>> On 7/1/20 7:39 AM, Szabolcs Nagy wrote:
>>>> +  unsigned prot = PROT_READ | PROT_EXEC | PROT_BTI;
>>>> +
>>>> +  for (phdr = map->l_phdr; phdr < &map->l_phdr[map->l_phnum]; ++phdr)
>>>> +    if (phdr->p_type == PT_LOAD && (phdr->p_flags & PF_X))
>>>> +      {
>>>> +	ElfW(Addr) start = phdr->p_vaddr + map->l_addr;
>>>> +	ElfW(Addr) len = phdr->p_memsz;
>>>> +	if (__mprotect ((void *) start, len, prot) < 0)
>>>
>>> Spend an extra cycle or two and honor the exact set of p_flags?
>>>
>>> If I construct an executable but non-readable PT_LOAD segment, that should be
>>> fine.  RWX with BTI would undoubtedly be an unusual case, but perhaps not
>>> implausible in the context of a JIT with a statically allocated buffer.
>>
>> i will commit the attached fix tomorrow
>> unless there are comments on it.

Thanks.  LGTM, modulo the naming typo.


r~

>>
> 
>> From ee5765a839f6a40a61960264ed46393dc5d6c534 Mon Sep 17 00:00:00 2001
>> From: Szabolcs Nagy <szabolcs.nagy@arm.com>
>> Date: Mon, 13 Jul 2020 11:28:18 +0100
>> Subject: [PATCH] aarch64: Respect p_flags when protecting code with PROT_BTI
>>
>> Use PROT_READ and PROT_WRITE according to the load segment p_flags
>> when adding PROT_BTI.
>>
>> This is before processing relocations which may drop PROT_BTI in
>> case of textrels.  Executable stacks are not protected via PROT_BTI
>> either.  PROT_BTI is hardening in case memory corruption happened,
>> it's value is reduced if there is writable and executable memory
>> available so missing it on such memory is fine, but we should
>> respect the p_flags and should not drop PROT_WRITE.
>> ---
>>  sysdeps/aarch64/dl-bti.c | 9 ++++++++-
>>  1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/sysdeps/aarch64/dl-bti.c b/sysdeps/aarch64/dl-bti.c
>> index 965ddcc732..01ffb69a4a 100644
>> --- a/sysdeps/aarch64/dl-bti.c
>> +++ b/sysdeps/aarch64/dl-bti.c
>> @@ -24,13 +24,20 @@ static int
>>  enable_bti (struct link_map *map, const char *program)
>>  {
>>    const ElfW(Phdr) *phdr;
>> -  unsigned prot = PROT_READ | PROT_EXEC | PROT_BTI;
>> +  unsigned prot;
>>  
>>    for (phdr = map->l_phdr; phdr < &map->l_phdr[map->l_phnum]; ++phdr)
>>      if (phdr->p_type == PT_LOAD && (phdr->p_flags & PF_X))
>>        {
>>  	void *start = (void *) (phdr->p_vaddr + map->l_addr);
>>  	size_t len = phdr->p_memsz;
>> +
>> +	prot = PROT_EXEC | PROT_BTI;
>> +	if (ph->p_flags & PF_R)
> 
> should be phdr-> instead of ph->
> 
> and i will have to wait for the tests to finish too which
> takes about 40h.
> 
>> +	  prot |= PROT_READ;
>> +	if (ph->p_flags & PF_W)
>> +	  prot |= PROT_WRITE;
>> +
>>  	if (__mprotect (start, len, prot) < 0)
>>  	  {
>>  	    if (program)
>> -- 
>> 2.17.1
>>