MMU ↔ PAN: Does PAN alone prevent accidental public access to user memory?

Question

MMU ↔ PAN Integration
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.
Unresolved Behavior
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:

Can PAN alone guarantee that a kernel cannot read user‑space memory, or must PXN bits be set on every page table entry?
How does MTE affect the reliability of PAN in preventing accidental public access?

Tasadduq Burney · Answer

Direct Resolution
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.

Mechanism Explanation
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:

MMU Page Tables: These define the memory map. If a page is marked as privileged-only, the user cannot access it. If it is marked as user-accessible, the kernel (by default) can still access it because privileged mode generally inherits all permissions of unprivileged mode.
PAN (PSTATE.PAN): This bit tells the hardware: "Even though I am in privileged mode, do not allow me to access any page that is marked as user-accessible unless I explicitly toggle this bit off."

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 Role of PXN and MTE
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.

Verification Steps
To verify the current enforcement state on an ARMv8.1-A+ system, you can perform the following checks:

Hardware Support: Check if the CPU supports PAN via the system registers or kernel boot logs.
Kernel Implementation: In the Linux kernel, observe the use of copy_from_user() and copy_to_user(). These functions temporarily disable PAN to allow legitimate data transfers between user and kernel space.
Page Table Audit: Ensure that user-space pages are correctly flagged with the 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.

MMU ↔ PAN: Does PAN alone prevent accidental public access to user memory?

MMU ↔ PAN Integration

Unresolved Behavior

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Direct Resolution

Mechanism Explanation

The Role of PXN and MTE

Verification Steps

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