From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B52CC385D85; Tue, 12 May 2026 17:44:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778607851; cv=none; b=Zxy6Fgj7saLyESn6xSJuQNZLM5RlJX4uAXdJCW6dIthUwgbkKsaZoJUi265Bo0fTzXeSeS9nLXtcUC0NTHnBK5xGgzertKMrGWwnVrK1YScOB4VbHpyserBPY6PxD4LjgAMZKB4N19ej2FC2B3NFxFooFKXF27qM+9boSlVNaRs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778607851; c=relaxed/simple; bh=a7pnDQBddgO+VlUoE4BD0uai40YUmqRxGh4G9ueWWU4=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=XopZmgswIVEy9EbYY93Q5LsEStDnirCsY/8jbBwXtzMvFY7SJ16BCLnDjhiJ/JWL6TRo4Kt75Z3bFwugeo9jHujgsohEToInZJEt6xSpOr5wU+DZFzzpxtqRydVBs0VHAh+3iFldi4xqKMqStG5Ist3NcKxGXFMTRImeLHE1Caw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=mXXgDJ8j; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="mXXgDJ8j" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D3F5C2BCB0; Tue, 12 May 2026 17:44:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1778607851; bh=a7pnDQBddgO+VlUoE4BD0uai40YUmqRxGh4G9ueWWU4=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=mXXgDJ8jMUkyJ5hzqP3YXqoQN/0gHl7z6iLji5lyyZ4RJN2COXuIFqcSamVE84u9y xBXQ1gsxPv5FCaIdnKMcIagT+oEfzdG4hpLtqVCBW4ykQcvlvp7hxT+QtmGoMNaE/7 39br/vRxREB7jS0m2LPrL9o1sho5EFWjcyj3it+k= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Rick Edgecombe , Sasha Levin Subject: [PATCH 6.12 016/206] x86/shstk: Prevent deadlock during shstk sigreturn Date: Tue, 12 May 2026 19:37:48 +0200 Message-ID: <20260512173933.164829250@linuxfoundation.org> X-Mailer: git-send-email 2.54.0 In-Reply-To: <20260512173932.810559588@linuxfoundation.org> References: <20260512173932.810559588@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Rick Edgecombe [ Upstream commit 9874b2917b9fbc30956fee209d3c4aa47201c64e ] During sigreturn the shadow stack signal frame is popped. The kernel does this by reading the shadow stack using normal read accesses. When it can't assume the memory is shadow stack, it takes extra steps to makes sure it is reading actual shadow stack memory and not other normal readable memory. It does this by holding the mmap read lock while doing the access and checking the flags of the VMA. Unfortunately that is not safe. If the read of the shadow stack sigframe hits a page fault, the fault handler will try to recursively grab another mmap read lock. This normally works ok, but if a writer on another CPU is also waiting, the second read lock could fail and cause a deadlock. Fix this by doing the read of the userspace memory via gup. Embed it in the get_shstk_data() helper. Currently there is a check that skips the lookup work when the SSP can be assumed to be on a shadow stack. While reorganizing the function, remove the optimization to make the tricky code flows more common, such that issues like this cannot escape detection for so long. [Due to missing per-vma MM sequence counter, use a simpler GUP based solution for the backport] Cc: # Depends on https://lore.kernel.org/all/20260504205856.536296-1-rick.p.edgecombe@intel.com/ Signed-off-by: Rick Edgecombe Signed-off-by: Sasha Levin --- arch/x86/kernel/shstk.c | 46 ++++++++++++++++++++++++++--------------- 1 file changed, 29 insertions(+), 17 deletions(-) diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c index 0dc983b33b003..373a44a5c478f 100644 --- a/arch/x86/kernel/shstk.c +++ b/arch/x86/kernel/shstk.c @@ -18,6 +18,7 @@ #include #include #include +#include #include #include #include @@ -262,11 +263,29 @@ static int put_shstk_data(u64 __user *addr, u64 data) return 0; } +/* Copy from aligned address in userspace without risk of page fault. */ +static int shstk_copy_user_gup(unsigned long *ldata, unsigned long __user *addr) +{ + struct page *page; + void *kaddr; + + mmap_assert_locked(current->mm); + if (get_user_pages((unsigned long)addr, 1, 0, &page) != 1) + return -EFAULT; + + kaddr = kmap_local_page(page); + *ldata = *(unsigned long *)(kaddr + offset_in_page(addr)); + kunmap_local(kaddr); + put_page(page); + + return 0; +} + static int get_shstk_data(unsigned long *data, unsigned long __user *addr) { unsigned long ldata; - if (unlikely(get_user(ldata, addr))) + if (shstk_copy_user_gup(&ldata, addr)) return -EFAULT; if (!(ldata & SHSTK_DATA_BIT)) @@ -296,7 +315,6 @@ static int shstk_pop_sigframe(unsigned long *ssp) { struct vm_area_struct *vma; unsigned long token_addr; - bool need_to_check_vma; int err = 1; /* @@ -308,26 +326,21 @@ static int shstk_pop_sigframe(unsigned long *ssp) if (!IS_ALIGNED(*ssp, 8)) return -EINVAL; - need_to_check_vma = PAGE_ALIGN(*ssp) == *ssp; - - if (need_to_check_vma) - if (mmap_read_lock_killable(current->mm)) - return -EINTR; + if (mmap_read_lock_killable(current->mm)) + return -EINTR; err = get_shstk_data(&token_addr, (unsigned long __user *)*ssp); if (unlikely(err)) goto out_err; - if (need_to_check_vma) { - vma = find_vma(current->mm, *ssp); - if (!vma || !(vma->vm_flags & VM_SHADOW_STACK)) { - err = -EFAULT; - goto out_err; - } - - mmap_read_unlock(current->mm); + vma = find_vma(current->mm, *ssp); + if (!vma || !(vma->vm_flags & VM_SHADOW_STACK)) { + err = -EFAULT; + goto out_err; } + mmap_read_unlock(current->mm); + /* Restore SSP aligned? */ if (unlikely(!IS_ALIGNED(token_addr, 8))) return -EINVAL; @@ -340,8 +353,7 @@ static int shstk_pop_sigframe(unsigned long *ssp) return 0; out_err: - if (need_to_check_vma) - mmap_read_unlock(current->mm); + mmap_read_unlock(current->mm); return err; } -- 2.53.0