| Message ID | 20180124175631.22925-6-igor.stoppa@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
2 Minor typos inline below: On 01/24/2018 09:56 AM, Igor Stoppa wrote: > Detailed documentation about the protectable memory allocator. > > Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com> > --- > Documentation/core-api/pmalloc.txt | 104 +++++++++++++++++++++++++++++++++++++ > 1 file changed, 104 insertions(+) > create mode 100644 Documentation/core-api/pmalloc.txt > > diff --git a/Documentation/core-api/pmalloc.txt b/Documentation/core-api/pmalloc.txt > new file mode 100644 > index 0000000..9c39672 > --- /dev/null > +++ b/Documentation/core-api/pmalloc.txt > @@ -0,0 +1,104 @@ > +============================ > +Protectable memory allocator > +============================ > + > +Introduction > +------------ > + > +When trying to perform an attack toward a system, the attacker typically > +wants to alter the execution flow, in a way that allows actions which > +would otherwise be forbidden. > + > +In recent years there has been lots of effort in preventing the execution > +of arbitrary code, so the attacker is progressively pushed to look for > +alternatives. > + > +If code changes are either detected or even prevented, what is left is to > +alter kernel data. > + > +As countermeasure, constant data is collected in a section which is then > +marked as readonly. > +To expand on this, also statically allocated variables which are tagged > +as __ro_after_init will receive a similar treatment. > +The difference from constant data is that such variables can be still > +altered freely during the kernel init phase. > + > +However, such solution does not address those variables which could be > +treated essentially as read-only, but whose size is not known at compile > +time or cannot be fully initialized during the init phase. > + > + > +Design > +------ > + > +pmalloc builds on top of genalloc, using the same concept of memory pools > +A pool is a handle to a group of chunks of memory of various sizes. > +When created, a pool is empty. It will be populated by allocating chunks > +of memory, either when the first memory allocation request is received, or > +when a pre-allocation is performed. > + > +Either way, one or more memory pages will be obtaiend from vmalloc and obtained > +registered in the pool as chunk. Subsequent requests will be satisfied by > +either using any available free space from the current chunks, or by > +allocating more vmalloc pages, should the current free space not suffice. > + > +This is the key point of pmalloc: it groups data that must be protected > +into a set of pages. The protection is performed through the mmu, which > +is a prerequisite and has a minimum granularity of one page. > + > +If the relevant variables were not grouped, there would be a problem of > +allowing writes to other variables that might happen to share the same > +page, but require further alterations over time. > + > +A pool is a group of pages that are write protected at the same time. > +Ideally, they have some high level correlation (ex: they belong to the > +same module), which justifies write protecting them all together. > + > +To keep it to a minimum, locking is left to the user of the API, in > +those cases where it's not strictly needed. > +Ideally, no further locking is required, since each module can have own > +pool (or pools), which should, for example, avoid the need for cross > +module or cross thread synchronization about write protecting a pool. > + > +The overhead of creating an additional pool is minimal: a handful of bytes > +from kmalloc space for the metadata and then what is left unused from the > +page(s) registered as chunks. > + > +Compared to plain use of vmalloc, genalloc has the advantage of tightly > +packing the allocations, reducing the number of pages used and therefore > +the pressure on the TLB. The slight overhead in execution time of the > +allocation should be mostly irrelevant, because pmalloc memory is not > +meant to be allocated/freed in tight loops. Rather it ought to be taken > +in use, initialized and write protected. Possibly destroyed. > + > +Considering that not much data is supposed to be dynamically allocated > +and then marked as read-only, it shouldn't be an issue that the address > +range for pmalloc is limited, on 32-bit systemd. > + > +Regarding SMP systems, the allocations are expected to happen mostly > +during an initial transient, after which there should be no more need to > +perform cross-processor synchronizations of page tables. > + > + > +Use > +--- > + > +The typical sequence, when using pmalloc, is: > + > +1. create a pool > +2. [optional] pre-allocate some memory in the pool > +3. issue one or more allocation requests to the pool > +4. initialize the memory obtained > + - iterate over points 3 & 4 as needed - > +5. write protect the pool > +6. use in read-only mode the handlers obtained throguh the allocations through > +7. [optional] destroy the pool > + > + > +In a scenario where, for example due to some error, part or all of the > +allocations performed at point 3 must be reverted, it is possible to free > +them, as long as point 5 has not been executed, and the pool is still > +modifiable. Such freed memory can be re-used. > +Performing a free operation on a write-protected pool will, instead, > +simply release the corresponding memory from the accounting, but it will > +be still impossible to alter its content. >
On 24/01/18 21:14, Ralph Campbell wrote:
> 2 Minor typos inline below:
thanks for proof-reading, will fix accordingly.
--
igor
diff --git a/Documentation/core-api/pmalloc.txt b/Documentation/core-api/pmalloc.txt new file mode 100644 index 0000000..9c39672 --- /dev/null +++ b/Documentation/core-api/pmalloc.txt @@ -0,0 +1,104 @@ +============================ +Protectable memory allocator +============================ + +Introduction +------------ + +When trying to perform an attack toward a system, the attacker typically +wants to alter the execution flow, in a way that allows actions which +would otherwise be forbidden. + +In recent years there has been lots of effort in preventing the execution +of arbitrary code, so the attacker is progressively pushed to look for +alternatives. + +If code changes are either detected or even prevented, what is left is to +alter kernel data. + +As countermeasure, constant data is collected in a section which is then +marked as readonly. +To expand on this, also statically allocated variables which are tagged +as __ro_after_init will receive a similar treatment. +The difference from constant data is that such variables can be still +altered freely during the kernel init phase. + +However, such solution does not address those variables which could be +treated essentially as read-only, but whose size is not known at compile +time or cannot be fully initialized during the init phase. + + +Design +------ + +pmalloc builds on top of genalloc, using the same concept of memory pools +A pool is a handle to a group of chunks of memory of various sizes. +When created, a pool is empty. It will be populated by allocating chunks +of memory, either when the first memory allocation request is received, or +when a pre-allocation is performed. + +Either way, one or more memory pages will be obtaiend from vmalloc and +registered in the pool as chunk. Subsequent requests will be satisfied by +either using any available free space from the current chunks, or by +allocating more vmalloc pages, should the current free space not suffice. + +This is the key point of pmalloc: it groups data that must be protected +into a set of pages. The protection is performed through the mmu, which +is a prerequisite and has a minimum granularity of one page. + +If the relevant variables were not grouped, there would be a problem of +allowing writes to other variables that might happen to share the same +page, but require further alterations over time. + +A pool is a group of pages that are write protected at the same time. +Ideally, they have some high level correlation (ex: they belong to the +same module), which justifies write protecting them all together. + +To keep it to a minimum, locking is left to the user of the API, in +those cases where it's not strictly needed. +Ideally, no further locking is required, since each module can have own +pool (or pools), which should, for example, avoid the need for cross +module or cross thread synchronization about write protecting a pool. + +The overhead of creating an additional pool is minimal: a handful of bytes +from kmalloc space for the metadata and then what is left unused from the +page(s) registered as chunks. + +Compared to plain use of vmalloc, genalloc has the advantage of tightly +packing the allocations, reducing the number of pages used and therefore +the pressure on the TLB. The slight overhead in execution time of the +allocation should be mostly irrelevant, because pmalloc memory is not +meant to be allocated/freed in tight loops. Rather it ought to be taken +in use, initialized and write protected. Possibly destroyed. + +Considering that not much data is supposed to be dynamically allocated +and then marked as read-only, it shouldn't be an issue that the address +range for pmalloc is limited, on 32-bit systemd. + +Regarding SMP systems, the allocations are expected to happen mostly +during an initial transient, after which there should be no more need to +perform cross-processor synchronizations of page tables. + + +Use +--- + +The typical sequence, when using pmalloc, is: + +1. create a pool +2. [optional] pre-allocate some memory in the pool +3. issue one or more allocation requests to the pool +4. initialize the memory obtained + - iterate over points 3 & 4 as needed - +5. write protect the pool +6. use in read-only mode the handlers obtained throguh the allocations +7. [optional] destroy the pool + + +In a scenario where, for example due to some error, part or all of the +allocations performed at point 3 must be reverted, it is possible to free +them, as long as point 5 has not been executed, and the pool is still +modifiable. Such freed memory can be re-used. +Performing a free operation on a write-protected pool will, instead, +simply release the corresponding memory from the accounting, but it will +be still impossible to alter its content.
Detailed documentation about the protectable memory allocator. Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com> --- Documentation/core-api/pmalloc.txt | 104 +++++++++++++++++++++++++++++++++++++ 1 file changed, 104 insertions(+) create mode 100644 Documentation/core-api/pmalloc.txt