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riscv: avoid undefined access in lookupPTSlot

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The function getPPtrFromHWPTE is not applicable to invalid PTEs, so
this commit refactors the loop such that the the access is guaranteed
to be guarded by the corresponding isPTEPageTable check.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>
This commit is contained in:
Gerwin Klein 2020-04-19 14:56:20 +08:00
parent 1fba3d9e68
commit 9181beb963
1 changed files with 12 additions and 10 deletions
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22
src/arch/riscv/kernel/vspace.c
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@ -372,22 +372,24 @@ static inline pte_t *getPPtrFromHWPTE(pte_t *pte)
lookupPTSlot_ret_t lookupPTSlot(pte_t *lvl1pt, vptr_t vptr)
{
lookupPTSlot_ret_t ret;
word_t level = CONFIG_PT_LEVELS - 1;
pte_t *pt = lvl1pt;
/* this is how many bits we potentially have left to decode. Initially we have the
* full address space to decode, and every time we walk this will be reduced. The
* final value of this after the walk is the size of the frame that can be inserted,
* or already exists, in ret.ptSlot */
ret.ptBitsLeft = PT_INDEX_BITS * CONFIG_PT_LEVELS + seL4_PageBits;
ret.ptSlot = NULL;
* or already exists, in ret.ptSlot. The following formulation is an invariant of
* the loop: */
ret.ptBitsLeft = PT_INDEX_BITS * level + seL4_PageBits;
ret.ptSlot = pt + ((vptr >> ret.ptBitsLeft) & MASK(PT_INDEX_BITS));
pte_t *pt = lvl1pt;
do {
while (isPTEPageTable(ret.ptSlot) && likely(0 < level)) {
level--;
ret.ptBitsLeft -= PT_INDEX_BITS;
word_t index = (vptr >> ret.ptBitsLeft) & MASK(PT_INDEX_BITS);
ret.ptSlot = pt + index;
pt = getPPtrFromHWPTE(ret.ptSlot);
/* stop when we find something that isn't a page table - either a mapped frame or
* an empty slot */
} while (isPTEPageTable(ret.ptSlot));
ret.ptSlot = pt + ((vptr >> ret.ptBitsLeft) & MASK(PT_INDEX_BITS));
}
return ret;
}