x86 restore_ucontext(): validate user-modified contex values#2144
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francescolavra merged 2 commits intomasterfrom Mar 7, 2026
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x86 restore_ucontext(): validate user-modified contex values#2144francescolavra merged 2 commits intomasterfrom
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niklasfemerstrand
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March 7, 2026 08:33
A non-canonical instruction pointer value (i.e. a value where bit 47 is not sign-extended to 64 bits) causes a general protection fault. If a signal handler writes a non-canonical RIP value to the machine context, the sysret instruction executed by the kernel when the handler returns triggers the general protection fault in kernel mode (on Intel CPUs). To avoid faulting in kernel mode, only copy the RIP value from the machine context to the thread frame if the value is canonical.
…rame A signal handler can modify the pointer to the floating-point register frame in the thread context. If a modified pointer is invalid, the kernel crashes; if a modified pointer points to kernel memory, kernel info is leaked to the user program. To prevent these issues, avoid copying from non-legitimate addresses when restoring the thread context after a signal handler.
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A signal handler can modify the machine context that is applied to the thread frame when the handler returns. The kernel should validate any user-modified values that might cause faults in kernel mode or leak kernel info to the user program.
Issues reported and fixes suggested by Niklas Femerstrand (@niklasfemerstrand).