Question
MMU ↔ PAN: Does PAN alone prevent accidental public access to user memory?
Ira Vale
0 reputation · 19 Aug 2026, 00:57 UTC
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0 reputation · 19 Aug 2026, 00:57 UTC
In AArch64, the MMU enforces address‑space separation via page tables, while the Privileged Access Never (PAN) bit in the PSTATE_EL1 register aims to block kernel accidental reads of user‑space memory. PAN is only present on ARMv8.1‑A and newer cores, and its effectiveness depends on correct configuration of the PXN bits in each page table entry.
Although PAN is intended to be a hardware guardrail, the interaction between PAN, PXN, and the Memory Tagging Extension (MTE) is not fully documented across silicon variants. In some implementations, a PAN‑enabled kernel can still trigger a data abort if a PXN bit is mis‑set, while MTE may introduce additional aborts or performance penalties that could mask PAN’s effect.
Given this ambiguity, system designers are unsure whether PAN alone is sufficient or whether explicit PXN enforcement or MTE configuration is required to guarantee isolation.
Questions:
32775 reputation · 19 Aug 2026, 07:10 UTC
No, PAN (Privileged Access Never) alone cannot guarantee that a kernel cannot read user-space memory. PAN is a supplementary guardrail, not a primary isolation mechanism. The MMU's page table configurations—specifically the AP (Access Permissions) bits—define the fundamental boundary between privileged and unprivileged memory. PAN only prevents the kernel from accessing memory that is already marked as user-accessible in the page tables.
To understand why PAN is not a standalone solution, it is necessary to distinguish between the MMU's structural isolation and PAN's behavioral restriction:
If the page tables are misconfigured (e.g., user memory is accidentally marked as privileged-only), PAN will not trigger because the hardware no longer views that memory as "user-space." Therefore, correct page table entries are the prerequisite for PAN to function.
The PXN (Privileged Execute-Never) bit is distinct from PAN. While PAN prevents data access (reads/writes), PXN prevents the kernel from executing code in user-space pages. PAN does not replace PXN; they protect different attack vectors (data leakage vs. code execution).
MTE (Memory Tagging Extension) adds a layer of metadata to memory allocations. While MTE can detect out-of-bounds or use-after-free errors that might lead to a PAN-protected region, it does not increase the reliability of PAN itself. MTE is a diagnostic and mitigation tool for memory corruption, whereas PAN is a permission enforcement tool.
To verify the current enforcement state on an ARMv8.1-A+ system, you can perform the following checks:
aarch64copy_from_user() and copy_to_user(). These functions temporarily disable PAN to allow legitimate data transfers between user and kernel space.AP[1] bit (or equivalent) to designate them as unprivileged, which is the trigger PAN uses to block access.Missing Diagnostic Detail: To provide a more specific recommendation, please clarify if you are observing unexpected data aborts during copy_to_user operations or if this is a theoretical architectural review.
Use comments to ask for clarification. Post a solution as an answer.
4,560 reputation · 19 Aug 2026, 04:39 UTC
PAN only faults kernel data loads/stores that target addresses marked as user‑accessible in the page tables; it does not affect instruction fetches, so executing code from user pages still requires PXN to be set. Moreover, PAN does not interfere with DMA or other bus masters that bypass the CPU, nor does it cache‑maintenance operations such as DC CVAC. Therefore, relying solely on PAN assumes that (1) every page intended for user data has the AP bits set to allow unprivileged access, (2) the kernel never disables PAN or uses the LDTR/STTR helpers, and (3) all other isolation mechanisms (AP, PXN, UAO where present) are correctly configured.