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uprobes: sanitiize xol_free_insn_slot()

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1. Clear utask->xol_vaddr unconditionally, even if this addr is not valid,
   xol_free_insn_slot() should never return with utask->xol_vaddr != NULL.

2. Add a comment to explain why do we need to validate slot_addr.

3. Simplify the validation above. We can simply check offset < PAGE_SIZE,
   unsigned underflows are fine, it should work if slot_addr < area->vaddr.

4. Kill the unnecessary "slot_nr >= UINSNS_PER_PAGE" check, slot_nr must
   be valid if offset < PAGE_SIZE.

The next patches will cleanup this function even more.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240929144235.GA9471@redhat.com
This commit is contained in:
Oleg Nesterov 2024-09-29 16:42:35 +02:00 committed by Peter Zijlstra
parent b302d5a6ff
commit c7b4133c48
1 changed files with 9 additions and 12 deletions
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21
kernel/events/uprobes.c
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@ -1680,8 +1680,8 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
static void xol_free_insn_slot(struct task_struct *tsk)
{
struct xol_area *area;
unsigned long vma_end;
unsigned long slot_addr;
unsigned long offset;
if (!tsk->mm || !tsk->mm->uprobes_state.xol_area || !tsk->utask)
return;
@ -1690,24 +1690,21 @@ static void xol_free_insn_slot(struct task_struct *tsk)
if (unlikely(!slot_addr))
return;
tsk->utask->xol_vaddr = 0;
area = tsk->mm->uprobes_state.xol_area;
vma_end = area->vaddr + PAGE_SIZE;
if (area->vaddr <= slot_addr && slot_addr < vma_end) {
unsigned long offset;
int slot_nr;
offset = slot_addr - area->vaddr;
slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
if (slot_nr >= UINSNS_PER_PAGE)
return;
offset = slot_addr - area->vaddr;
/*
* slot_addr must fit into [area->vaddr, area->vaddr + PAGE_SIZE).
* This check can only fail if the "[uprobes]" vma was mremap'ed.
*/
if (offset < PAGE_SIZE) {
int slot_nr = offset / UPROBE_XOL_SLOT_BYTES;
clear_bit(slot_nr, area->bitmap);
atomic_dec(&area->slot_count);
smp_mb__after_atomic(); /* pairs with prepare_to_wait() */
if (waitqueue_active(&area->wq))
wake_up(&area->wq);
tsk->utask->xol_vaddr = 0;
}
}