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SubjectRe: [RFC][PATCH 5/8] x86/mm: fix exception table comments
On Sat, Sep 8, 2018 at 2:22 AM Dave Hansen <dave.hansen@linux.intel.com> wrote:
> + * Kernel-mode access to the user address space should only occur
> + * inside well-defined areas of code listed in the exception

Actually, not areas, but single whitelisted instructions. It would
probably be nice to say that more clearly.

From arch/x86/include/asm/extable.h:

/*
* The exception table consists of triples of addresses relative to the
* exception table entry itself. The first address is of an instruction
* that is allowed to fault, the second is the target at which the program
* should continue. The third is a handler function to deal with the fault
* caused by the instruction in the first field.
*
* All the routines below use bits of fixup code that are out of line
* with the main instruction path. This means when everything is well,
* we don't even have to jump over them. Further, they do not intrude
* on our cache or tlb entries.
*/

struct exception_table_entry {
int insn, fixup, handler;
};

> + * tables. But, an erroneous kernel fault occurring outside one of
> + * those areas which also holds mmap_sem might deadlock attempting
> + * to validate the fault against the address space.
> *
> - * As the vast majority of faults will be valid we will only perform
> - * the source reference check when there is a possibility of a
> - * deadlock. Attempt to lock the address space, if we cannot we then
> - * validate the source. If this is invalid we can skip the address
> - * space check, thus avoiding the deadlock:
> + * Only do the expensive exception table search when we might be at
> + * risk of a deadlock:
> + * 1. We failed to acquire mmap_sem, and
> + * 2. The access was an explicit kernel-mode access
> + * (X86_PF_USER=0).
> */
> if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
> if (!(sw_error_code & X86_PF_USER) &&
> !search_exception_tables(regs->ip)) {
> + /*
> + * Fault from code in kernel from
> + * which we do not expect faults.
> + */
> bad_area_nosemaphore(regs, sw_error_code, address, NULL);
> return;
> }
> _
>

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