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9c4bbee9e3
If multiple guest threads in user-mode emulation write to a page which QEMU has marked read-only because of cached TCG translations, the threads can race in page_unprotect: * threads A & B both try to do a write to a page with code in it at the same time (ie which we've made non-writeable, so SEGV) * they race into the signal handler with this faulting address * thread A happens to get to page_unprotect() first and takes the mmap lock, so thread B sits waiting for it to be done * A then finds the page, marks it PAGE_WRITE and mprotect()s it writable * A can then continue OK (returns from signal handler to retry the memory access) * ...but when B gets the mmap lock it finds that the page is already PAGE_WRITE, and so it exits page_unprotect() via the "not due to protected translation" code path, and wrongly delivers the signal to the guest rather than just retrying the access In particular, this meant that trying to run 'javac' in user-mode emulation would fail with a spurious guest SIGSEGV. Handle this by making page_unprotect() assume that a call for a page which is already PAGE_WRITE is due to a race of this sort and return a "fault handled" indication. Since this would cause an infinite loop if we ever called page_unprotect() for some other kind of fault than "write failed due to bad access permissions", tighten the condition in handle_cpu_signal() to check the signal number and si_code, and add a comment so that if somebody does ever find themselves debugging an infinite loop of faults they have some clue about why. (The trick for identifying the correct setting for current_tb_invalidated for thread B (needed to handle the precise-SMC case) is due to Richard Henderson. Paolo Bonzini suggested just relying on si_code rather than trying anything more complicated.) Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Message-Id: <1511879725-9576-3-git-send-email-peter.maydell@linaro.org> Signed-off-by: Laurent Vivier <laurent@vivier.eu> |
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| .. | ||
| atomic_template.h | ||
| cpu-exec-common.c | ||
| cpu-exec.c | ||
| cputlb.c | ||
| Makefile.objs | ||
| softmmu_template.h | ||
| tcg-all.c | ||
| tcg-runtime.c | ||
| tcg-runtime.h | ||
| trace-events | ||
| translate-all.c | ||
| translate-all.h | ||
| translator.c | ||
| user-exec-stub.c | ||
| user-exec.c | ||