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Mon, 04 Nov 2024 15:17:22 -0800 (PST) Received: from localhost ([2a00:79e0:9d:4:1b91:d97b:f7c9:2b60]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-431bd9aa611sm197561295e9.34.2024.11.04.15.17.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 04 Nov 2024 15:17:21 -0800 (PST) From: Jann Horn Date: Tue, 05 Nov 2024 00:17:13 +0100 Subject: [PATCH] drm/panthor: Be stricter about IO mapping flags Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20241105-panthor-flush-page-fixes-v1-1-829aaf37db93@google.com> X-B4-Tracking: v=1; b=H4sIAPhVKWcC/x2LQQqAIBAAvxJ7TlDxEH0lOmStuRAabkUg/r2l4 zAzFRgLIcPYVSj4EFNOAqbvYI1L2lHRJgxWW2eMdupc0hVzUeG4OQpJEehFVgGdRN6bYdUg+1n wF3JPc2sf9dqM/2oAAAA= X-Change-ID: 20241104-panthor-flush-page-fixes-fe4202bb18c0 To: Boris Brezillon , Steven Price , Liviu Dudau Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Jann Horn X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1730762237; l=5906; i=jannh@google.com; s=20240730; h=from:subject:message-id; bh=Ey3XFB5BJjjdu6VO458bb9nDOZIaqfSG6o++hBGILgU=; b=e1AET9ub1rPovwczIXEGiw8ak+NcFCdCvq/kFg8wlwHk3fGxhvKI5uyCuRaRAPvg0uDqRn9is 2oKkaV0l9iZAwspC9DjDNRDQgflyjOga6roZX1CpGToeKDiAzc2BqoQ X-Developer-Key: i=jannh@google.com; a=ed25519; pk=AljNtGOzXeF6khBXDJVVvwSEkVDGnnZZYqfWhP1V+C8= The current panthor_device_mmap_io() implementation has two issues: 1. For mapping DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET, panthor_device_mmap_io() bails if VM_WRITE is set, but does not clear VM_MAYWRITE. That means userspace can use mprotect() to make the mapping writable later on. This is a classic Linux driver gotcha. I don't think this actually has any impact in practice: When the GPU is powered, writes to the FLUSH_ID seem to be ignored; and when the GPU is not powered, the dummy_latest_flush page provided by the driver is deliberately designed to not do any flushes, so the only thing writing to the dummy_latest_flush could achieve would be to make *more* flushes happen. 2. panthor_device_mmap_io() does not block MAP_PRIVATE mappings (which are mappings without the VM_SHARED flag). MAP_PRIVATE in combination with VM_MAYWRITE indicates that the VMA has copy-on-write semantics, which for VM_PFNMAP are semi-supported but fairly cursed. In particular, in such a mapping, the driver can only install PTEs during mmap() by calling remap_pfn_range() (because remap_pfn_range() wants to **store the physical address of the mapped physical memory into the vm_pgoff of the VMA**); installing PTEs later on with a fault handler (as panthor does) is not supported in private mappings, and so if you try to fault in such a mapping, vmf_insert_pfn_prot() splats when it hits a BUG() check. Fix it by clearing the VM_MAYWRITE flag (userspace writing to the FLUSH_ID doesn't make sense) and requiring VM_SHARED (copy-on-write semantics for the FLUSH_ID don't make sense). Reproducers for both scenarios are in the notes of my patch on the mailing list; I tested that these bugs exist on a Rock 5B machine. Note that I only compile-tested the patch, I haven't tested it; I don't have a working kernel build setup for the test machine yet. Please test it before applying it. Cc: stable@vger.kernel.org Fixes: 5fe909cae118 ("drm/panthor: Add the device logical block") Signed-off-by: Jann Horn Reviewed-by: Boris Brezillon Reviewed-by: Liviu Dudau Reviewed-by: Steven Price --- First testcase (can write to the FLUSH_ID): ``` typeof(x) __res =3D (x); \ if (__res =3D=3D (typeof(x))-1) \ err(1, "SYSCHK(" #x ")"); \ __res; \ }) int main(void) { int fd =3D SYSCHK(open(GPU_PATH, O_RDWR)); // sanity-check that PROT_WRITE+MAP_SHARED fails void *mmap_write_res =3D mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET); if (mmap_write_res =3D=3D MAP_FAILED) { perror("mmap() with PROT_WRITE+MAP_SHARED failed as expected"); } else { errx(1, "mmap() with PROT_WRITE+MAP_SHARED worked???"); } // make a PROT_READ+MAP_SHARED mapping, and upgrade it to writable void *mmio_page =3D SYSCHK(mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET)); SYSCHK(mprotect(mmio_page, 0x1000, PROT_READ|PROT_WRITE)); volatile uint32_t *flush_counter =3D (volatile uint32_t*)mmio_page; uint32_t last_old =3D -1; while (1) { uint32_t old_val =3D *flush_counter; *flush_counter =3D 1111; uint32_t new_val =3D *flush_counter; if (old_val !=3D last_old) printf("flush counter: old=3D%u, new=3D%u\n", old_val, new_val); last_old =3D old_val; } } ``` Second testcase (triggers BUG() splat): ``` typeof(x) __res =3D (x); \ if (__res =3D=3D (typeof(x))-1) \ err(1, "SYSCHK(" #x ")"); \ __res; \ }) int main(void) { int fd =3D SYSCHK(open(GPU_PATH, O_RDWR)); // make a PROT_READ+**MAP_PRIVATE** mapping void *ptr =3D SYSCHK(mmap(NULL, 0x1000, PROT_READ, MAP_PRIVATE, fd, DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET)); // trigger a read fault *(volatile char *)ptr; } ``` The second testcase splats like this: ``` [ 2918.411814] ------------[ cut here ]------------ [ 2918.411857] kernel BUG at mm/memory.c:2220! [ 2918.411955] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP [...] [ 2918.416147] CPU: 3 PID: 2934 Comm: private_user_fl Tainted: G = O 6.1.43-19-rk2312 #428a0a5e6 [ 2918.417043] Hardware name: Radxa ROCK 5B (DT) [ 2918.417464] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE= =3D--) [ 2918.418119] pc : vmf_insert_pfn_prot+0x40/0xe4 [ 2918.418567] lr : panthor_mmio_vm_fault+0xb0/0x12c [panthor] [...] [ 2918.425746] Call trace: [ 2918.425972] vmf_insert_pfn_prot+0x40/0xe4 [ 2918.426342] __do_fault+0x38/0x7c [ 2918.426648] __handle_mm_fault+0x404/0x6dc [ 2918.427018] handle_mm_fault+0x13c/0x18c [ 2918.427374] do_page_fault+0x194/0x33c [ 2918.427716] do_translation_fault+0x60/0x7c [ 2918.428095] do_mem_abort+0x44/0x90 [ 2918.428410] el0_da+0x40/0x68 [ 2918.428685] el0t_64_sync_handler+0x9c/0xf8 [ 2918.429067] el0t_64_sync+0x174/0x178 ``` --- drivers/gpu/drm/panthor/panthor_device.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/drivers/gpu/drm/panthor/panthor_device.c b/drivers/gpu/drm/pan= thor/panthor_device.c index 4082c8f2951dfdace7f73a24d6fe34e9e7f920eb..6fbff516c1c1f047fcb4dee17b8= 7d8263616dc0c 100644 --- a/drivers/gpu/drm/panthor/panthor_device.c +++ b/drivers/gpu/drm/panthor/panthor_device.c @@ -390,11 +390,15 @@ int panthor_device_mmap_io(struct panthor_device *ptd= ev, struct vm_area_struct * { u64 offset =3D (u64)vma->vm_pgoff << PAGE_SHIFT; =20 + if ((vma->vm_flags & VM_SHARED) =3D=3D 0) + return -EINVAL; + switch (offset) { case DRM_PANTHOR_USER_FLUSH_ID_MMIO_OFFSET: if (vma->vm_end - vma->vm_start !=3D PAGE_SIZE || (vma->vm_flags & (VM_WRITE | VM_EXEC))) return -EINVAL; + vm_flags_clear(vma, VM_MAYWRITE); =20 break; =20 --- base-commit: d78f0ee0406803cda8801fd5201746ccf89e5e4a change-id: 20241104-panthor-flush-page-fixes-fe4202bb18c0 --=20 Jann Horn