Pkru

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If supported by hardware, each mapped user linear address has an associated 4-bit protection key. A new per-thread PKRU hardware register determines, for each protection key, whether user-mode addresses with that protection key may be read or written. Only one key may apply to a given range at a time. The default protection key index is zero, it is used even if no key was explicitly assigned to the address, or if the key was removed. The protection prevents the system from accessing user addresses as well as the user applications.

Pkru

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If the value pointed to by access is zero after the call, pkru, no read or write permissions is granted for map- pings which are assigned the key keyidx. After a successfull call, the range has the specified key assigned, even if the key is zero and it did not change the page table entries. The system provides convenient library helpers pkru both the syscall and the instructions, pkru, described below.

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Memory protection consists in managing access rights of memory pages, either to avoid bugs or preventing malicious behavior. This is usually done through system calls, for instance with mprotect on Linux, because modification of the page table entries requires privileged access. MPK also referred as PKU for Protection Keys for Userspace is a userspace hardware mechanism to control page table permissions, it works by tagging memory pages with protection keys using 4 previously unused bits, in other words we can use up to 16 distinct keys to tag our pages. Once a page is tagged we can change its protection rights at will, from userspace. But, because updating page table entries PTE requires privileged access, a system call is still necessary to tag the pages with a given key in the first place. To allocate and free a key we also need to go through the kernel, on Linux the API is the following:. As I mentioned before there is only 16 keys available the key 0 being already used as a default to tag newly allocated pages , thus allocation can fail. You can learn more on the Linux API here. Thus, we can both read and write if the two bits WD, AD are set to 0, 0 , only read with 1, 0 and have no access with 0, 1 or 1, 1.

Pkru

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The key indexes written into the page table entries are managed by the sysarch syscall. Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error. Privacy policy. A new per-thread PKRU hardware register determines, for each protection key, whether user-mode addresses with that protection key may be read or written. Protection keys require that the system uses 4-level paging also called long mode , which means that it is only available on amd64 system. The mechanism gives safety measures which can be used to avoid incidental read or modification of sensitive memory, or as a debugging feature. Both bit and bit applications can use protection keys. Only one key may apply to a given range at a time. The protection prevents the system from accessing user addresses as well as the user applications. Existing mappings are changed to use key index zero in page table entries. Starting address is truncated to the page start, and the end is rounded up to the end of the page. They are re-established when the current mapping is destroyed and a new mapping is created in any sub-range of the specified range. The keys assigned to the range are persistent.

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Only assign the key if the range does not have any other keys assigned including the zero key. FreeBSD Privacy policy. It cannot guard against conscious accesses since permissions are user-controllable. They are re-estab- lished when the current mapping is destroyed and a new mapping is created in any sub-range of the specified range. Only one key may apply to a given range at a time. Both bit and bit applications can use protection keys. They are re-established when the current mapping is destroyed and a new mapping is created in any sub-range of the specified range. Existing mappings are changed to use key index zero in page table entries. After a successfull call, the range has the specified key assigned, even if the key is zero and it did not change the page table entries. Starting address is truncated to the page start, and the end is rounded up to the end of the page. The key indexes written into the page table entries are managed by the sysarch syscall. Peripheral Links.