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security.googleblog.com
Linux kernel support for the 32-bit ARM architecture was contributed in the late 90s, when there was little corporate involvement in Linux development, and most contributors were students or hobbyists, tinkering with development boards, often without much in the way of documentation.
Now 20+ years later, 32-bit ARM's maintainer has downgraded its support level to costs need to be kept to an absolute minimum. This is why 32-bit ARM is still widely used in embedded systems like set-top boxes and wireless routers.
Running 32-bit Linux on 64-bit ARM systems
Ironically, at these low price points, the DRAM is actually the dominant component in terms of BOM cost, and many of these 32-bit ARM systems incorporate a cheap ARMv8 that is used by other architectures to implement handling of silicon errata, which are particular to certain revisions of certain CPUs. Instead, on 32-bit multiplatform kernels, we simply enable all errata workarounds that may be needed by any of the cores that may ever run the image in question, potentially affecting performance unnecessarily on cores that have no need for them.Silicon vendors are phasing out 32-bit support in the longer term. Given an ecosystem containing a handful of operating systems and thousands of applications, support for 32-bit operating systems (which is more complex technically) is highly likely to be dropped first. For products with longer life cycles, long-term procurement contracts for components available today are usually much more costly than adjusting the BOM over time and using newer, cheaper parts. The 32-bit kernel does not implement or . Keeping the 32-bit ARM kernel secure
There are cases, though, where using the 32-bit kernel is the only option, e.g., if the CPUs are in fact 32-bit only (which is the case even for some ARMv8 cores such as Cortex-A32), or when relying on an existing 32-bit only codebase running in the kernel (drivers for legacy peripherals). Note that in such cases, it still makes sense to use the most recent kernel version compatible with the hardware, since we are in fact making an effort to enable some of the existing hardening features on 32-bit ARM as well.
Support for IRQ stacks
Coming up with a bounded worst case on which to base the size of the kernel stack is rather hard, especially given the fact that it is shared between the program itself and any exception handling routines that may be called on its behalf, including interrupt handlers. To mitigate the risk of a pathological worst case occurring, where an interrupt fires that needs a lot of stack space right at a time when most of the stack is already being used by the program, we are also in place, kernel stack overflow protection will be available for all ARM systems supported by Linux, including ancient ones like the Risc PC or
However, relying on legacy hardware and software comes with a risk, and even though we try to help keep users of the 32-bit kernel as safe as we reasonably can, it is not the right choice for new designs that incorporate 64-bit capable hardware.
Ironically, at these low price points, the DRAM is actually the dominant component in terms of BOM cost, and many of these 32-bit ARM systems incorporate a cheap ARMv8 that is used by other architectures to implement handling of silicon errata, which are particular to certain revisions of certain CPUs. Instead, on 32-bit multiplatform kernels, we simply enable all errata workarounds that may be needed by any of the cores that may ever run the image in question, potentially affecting performance unnecessarily on cores that have no need for them.
There are cases, though, where using the 32-bit kernel is the only option, e.g., if the CPUs are in fact 32-bit only (which is the case even for some ARMv8 cores such as Cortex-A32), or when relying on an existing 32-bit only codebase running in the kernel (drivers for legacy peripherals). Note that in such cases, it still makes sense to use the most recent kernel version compatible with the hardware, since we are in fact making an effort to enable some of the existing hardening features on 32-bit ARM as well.
- THREAD_INFO_IN_TASK for v7 SMP cores
The v5.16 release of the Linux kernel as well. When VMAP_STACK is enabled, kernel mode stacks are allocated from the kernel heap as before, but mapped into a different part of the kernel's address space, and surrounded by guard regions, which are guaranteed to be kept unpopulated. Given that accesses to such unpopulated regions will trigger an exception, the kernel's memory management layer can step in and terminate the program as soon as a stack overflow occurs, and prevent it from causing memory corruption.
Support for IRQ stacks
Coming up with a bounded worst case on which to base the size of the kernel stack is rather hard, especially given the fact that it is shared between the program itself and any exception handling routines that may be called on its behalf, including interrupt handlers. To mitigate the risk of a pathological worst case occurring, where an interrupt fires that needs a lot of stack space right at a time when most of the stack is already being used by the program, we are also in place, kernel stack overflow protection will be available for all ARM systems supported by Linux, including ancient ones like the Risc PC or
However, relying on legacy hardware and software comes with a risk, and even though we try to help keep users of the 32-bit kernel as safe as we reasonably can, it is not the right choice for new designs that incorporate 64-bit capable hardware.
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